diff --git a/README.md b/README.md
index 0b359294..c602fa92 100644
--- a/README.md
+++ b/README.md
@@ -21,11 +21,15 @@ Rhai's current features set:
 * Fairly efficient (1 million iterations in 0.75 sec on my 5 year old laptop)
 * Low compile-time overhead (~0.6 sec debug/~3 sec release for script runner app)
 * Relatively little `unsafe` code (yes there are some for performance reasons)
+* Sand-boxed (the scripting [`Engine`] can be declared immutable which cannot mutate the containing environment
+  unless explicitly allowed via `RefCell` etc.)
+* Rugged (protection against [stack-overflow](#maximum-stack-depth) and [runaway scripts](#maximum-number-of-operations))
+* Able to set limits on script resource usage (e.g. see [tracking progress](#tracking-progress))
 * [`no-std`](#optional-features) support
 * [Function overloading](#function-overloading)
 * [Operator overloading](#operator-overloading)
 * [Modules]
-* Compiled script is [optimized](#script-optimization) for repeat evaluations
+* Compiled script is [optimized](#script-optimization) for repeated evaluations
 * Support for [minimal builds](#minimal-builds) by excluding unneeded language [features](#optional-features)
 * Very few additional dependencies (right now only [`num-traits`](https://crates.io/crates/num-traits/)
   to do checked arithmetic operations); for [`no-std`](#optional-features) builds, a number of additional dependencies are
@@ -63,19 +67,19 @@ Beware that in order to use pre-releases (e.g. alpha and beta), the exact versio
 Optional features
 -----------------
 
-| Feature       | Description                                                                                                                           |
-| ------------- | ------------------------------------------------------------------------------------------------------------------------------------- |
-| `unchecked`   | Exclude arithmetic checking (such as overflows and division by zero). Beware that a bad script may panic the entire system!           |
-| `no_function` | Disable script-defined functions if not needed.                                                                                       |
-| `no_index`    | Disable [arrays] and indexing features if not needed.                                                                                 |
-| `no_object`   | Disable support for custom types and objects.                                                                                         |
-| `no_float`    | Disable floating-point numbers and math if not needed.                                                                                |
-| `no_optimize` | Disable the script optimizer.                                                                                                         |
-| `no_module`   | Disable modules.                                                                                                                      |
-| `only_i32`    | Set the system integer type to `i32` and disable all other integer types. `INT` is set to `i32`.                                      |
-| `only_i64`    | Set the system integer type to `i64` and disable all other integer types. `INT` is set to `i64`.                                      |
-| `no_std`      | Build for `no-std`. Notice that additional dependencies will be pulled in to replace `std` features.                                  |
-| `sync`        | Restrict all values types to those that are `Send + Sync`. Under this feature, [`Engine`], [`Scope`] and `AST` are all `Send + Sync`. |
+| Feature       | Description                                                                                                                                                                |
+| ------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `unchecked`   | Exclude arithmetic checking (such as over-flows and division by zero), stack depth limit and operations count limit. Beware that a bad script may panic the entire system! |
+| `no_function` | Disable script-defined functions if not needed.                                                                                                                            |
+| `no_index`    | Disable [arrays] and indexing features if not needed.                                                                                                                      |
+| `no_object`   | Disable support for custom types and objects.                                                                                                                              |
+| `no_float`    | Disable floating-point numbers and math if not needed.                                                                                                                     |
+| `no_optimize` | Disable the script optimizer.                                                                                                                                              |
+| `no_module`   | Disable modules.                                                                                                                                                           |
+| `only_i32`    | Set the system integer type to `i32` and disable all other integer types. `INT` is set to `i32`.                                                                           |
+| `only_i64`    | Set the system integer type to `i64` and disable all other integer types. `INT` is set to `i64`.                                                                           |
+| `no_std`      | Build for `no-std`. Notice that additional dependencies will be pulled in to replace `std` features.                                                                       |
+| `sync`        | Restrict all values types to those that are `Send + Sync`. Under this feature, [`Engine`], [`Scope`] and `AST` are all `Send + Sync`.                                      |
 
 By default, Rhai includes all the standard functionalities in a small, tight package.
 Most features are here to opt-**out** of certain functionalities that are not needed.
@@ -269,7 +273,7 @@ let ast = engine.compile_file("hello_world.rhai".into())?;
 
 ### Calling Rhai functions from Rust
 
-Rhai also allows working _backwards_ from the other direction - i.e. calling a Rhai-scripted function from Rust via `call_fn`.
+Rhai also allows working _backwards_ from the other direction - i.e. calling a Rhai-scripted function from Rust via `Engine::call_fn`.
 Functions declared with `private` are hidden and cannot be called from Rust (see also [modules]).
 
 ```rust
@@ -372,7 +376,7 @@ Use `Engine::new_raw` to create a _raw_ `Engine`, in which _nothing_ is added, n
 
 ### Packages
 
-Rhai functional features are provided in different _packages_ that can be loaded via a call to `load_package`.
+Rhai functional features are provided in different _packages_ that can be loaded via a call to `Engine::load_package`.
 Packages reside under `rhai::packages::*` and the trait `rhai::packages::Package` must be loaded in order for
 packages to be used.
 
@@ -759,7 +763,7 @@ Because they [_short-circuit_](#boolean-operators), `&&` and `||` are handled sp
 overriding them has no effect at all.
 
 Operator functions cannot be defined as a script function (because operators syntax are not valid function names).
-However, operator functions _can_ be registered to the [`Engine`] via `register_fn`, `register_result_fn` etc.
+However, operator functions _can_ be registered to the [`Engine`] via the methods `Engine::register_fn`, `Engine::register_result_fn` etc.
 When a custom operator function is registered with the same name as an operator, it _overloads_ (or overrides) the built-in version.
 
 ```rust
@@ -2043,7 +2047,7 @@ debug("world!");            // prints "world!" to stdout using debug formatting
 ### Overriding `print` and `debug` with callback functions
 
 When embedding Rhai into an application, it is usually necessary to trap `print` and `debug` output
-(for logging into a tracking log, for example).
+(for logging into a tracking log, for example) with the `Engine::on_print` and `Engine::on_debug` methods:
 
 ```rust
 // Any function or closure that takes an '&str' argument can be used to override
@@ -2237,7 +2241,7 @@ Built-in module resolvers are grouped under the `rhai::module_resolvers` module
 | `FileModuleResolver`   | The default module resolution service, not available under the [`no_std`] feature. Loads a script file (based off the current directory) with `.rhai` extension.<br/>The base directory can be changed via the `FileModuleResolver::new_with_path()` constructor function.<br/>`FileModuleResolver::create_module()` loads a script file and returns a module. |
 | `StaticModuleResolver` | Loads modules that are statically added. This can be used when the [`no_std`] feature is turned on.                                                                                                                                                                                                                                                            |
 
-An [`Engine`]'s module resolver is set via a call to `set_module_resolver`:
+An [`Engine`]'s module resolver is set via a call to `Engine::set_module_resolver`:
 
 ```rust
 // Use the 'StaticModuleResolver'
@@ -2248,6 +2252,112 @@ engine.set_module_resolver(Some(resolver));
 engine.set_module_resolver(None);
 ```
 
+Ruggedization - protect against DoS attacks
+------------------------------------------
+
+For scripting systems open to user-land scripts, it is always best to limit the amount of resources used by a script
+so that it does not crash the system by consuming all resources.
+
+The most important resources to watch out for are:
+
+* **Memory**: A malignant script may continuously grow an [array] or [object map] until all memory is consumed.
+* **CPU**: A malignant script may run an infinite tight loop that consumes all CPU cycles.
+* **Time**: A malignant script may run indefinitely, thereby blocking the calling system waiting for a result.
+* **Stack**: A malignant script may consume attempt an infinite recursive call that exhausts the call stack.
+* **Overflows**: A malignant script may deliberately cause numeric over-flows and/or under-flows, and/or bad
+  floating-point representations, in order to crash the system.
+* **Files**: A malignant script may continuously `import` an external module within an infinite loop,
+  thereby putting heavy load on the file-system (or even the network if the file is not local).
+* **Data**: A malignant script may attempt to read from and/or write to data that it does not own. If this happens,
+  it is a severe security breach and may put the entire system at risk.
+
+### Maximum number of operations
+
+Rhai by default does not limit how much time or CPU a script consumes.
+This can be changed via the `Engine::set_max_operations` method, with zero being unlimited (the default).
+
+```rust
+let mut engine = Engine::new();
+
+engine.set_max_operations(500);             // allow only up to 500 operations for this script
+
+engine.set_max_operations(0);               // allow unlimited operations
+```
+
+The concept of one single _operation_ in Rhai is volatile - it roughly equals one expression node,
+one statement, one iteration of a loop, or one function call etc. with sub-expressions and statement
+blocks executed inside these contexts accumulated on top.
+
+One _operation_ can take an unspecified amount of time and CPU cycles, depending on the particular operation
+involved.  For example, loading a constant consumes very few CPU cycles, while calling an external Rust function,
+though also counted as only one operation, may consume much more resources.
+
+The _operation count_ is intended to be a very course-grained measurement of the amount of CPU that a script
+is consuming, and allows the system to impose a hard upper limit.
+
+A script exceeding the maximum operations count will terminate with an error result.
+This check can be disabled via the [`unchecked`] feature for higher performance
+(but higher risks as well).
+
+### Tracking progress
+
+To track script evaluation progress and to force-terminate a script prematurely (for any reason),
+provide a closure to the `Engine::on_progress` method:
+
+```rust
+let mut engine = Engine::new();
+
+engine.on_progress(|count| {                // 'count' is the number of operations performed
+    if count % 1000 == 0 {
+        println!("{}", count);              // print out a progress log every 1,000 operations
+    }
+    true                                    // return 'true' to continue the script
+                                            // returning 'false' will terminate the script
+});
+```
+
+The closure passed to `Engine::on_progress` will be called once every operation.
+Return `false` to terminate the script immediately.
+
+### Maximum stack depth
+
+Rhai by default limits function calls to a maximum depth of 256 levels (28 levels in debug build).
+This limit may be changed via the `Engine::set_max_call_levels` method.
+The limit can be disabled via the [`unchecked`] feature for higher performance
+(but higher risks as well).
+
+```rust
+let mut engine = Engine::new();
+
+engine.set_max_call_levels(10);             // allow only up to 10 levels of function calls
+
+engine.set_max_call_levels(0);              // allow no function calls at all (max depth = zero)
+```
+
+A script exceeding the maximum call stack depth will terminate with an error result.
+
+### Checked arithmetic
+
+All arithmetic calculations in Rhai are _checked_, meaning that the script terminates with an error whenever
+it detects a numeric over-flow/under-flow condition or an invalid floating-point operation, instead of
+crashing the entire system.  This checking can be turned off via the [`unchecked`] feature for higher performance
+(but higher risks as well).
+
+### Access to external data
+
+Rhai is _sand-boxed_ so a script can never read from outside its own environment.
+Furthermore, an [`Engine`] created non-`mut` cannot mutate any state outside of itself;
+so it is highly recommended that [`Engine`]'s are created immutable as much as possible.
+
+```rust
+let mut engine = Engine::new();             // create mutable 'Engine'
+
+engine.register_get("add", add);            // configure 'engine'
+
+let engine = engine;                        // shadow the variable so that 'engine' is now immutable
+```
+
+
 Script optimization
 ===================
 
@@ -2358,7 +2468,7 @@ There are actually three levels of optimizations: `None`, `Simple` and `Full`.
 * `Full` is _much_ more aggressive, _including_ running functions on constant arguments to determine their result.
   One benefit to this is that many more optimization opportunities arise, especially with regards to comparison operators.
 
-An [`Engine`]'s optimization level is set via a call to `set_optimization_level`:
+An [`Engine`]'s optimization level is set via a call to `Engine::set_optimization_level`:
 
 ```rust
 // Turn on aggressive optimizations
diff --git a/src/api.rs b/src/api.rs
index 24b28474..78ceaf72 100644
--- a/src/api.rs
+++ b/src/api.rs
@@ -865,7 +865,7 @@ impl Engine {
         scope: &mut Scope,
         ast: &AST,
     ) -> Result<T, Box<EvalAltResult>> {
-        let result = self.eval_ast_with_scope_raw(scope, ast)?;
+        let (result, _) = self.eval_ast_with_scope_raw(scope, ast)?;
 
         let return_type = self.map_type_name(result.type_name());
 
@@ -881,7 +881,7 @@ impl Engine {
         &self,
         scope: &mut Scope,
         ast: &AST,
-    ) -> Result<Dynamic, Box<EvalAltResult>> {
+    ) -> Result<(Dynamic, u64), Box<EvalAltResult>> {
         let mut state = State::new(ast.fn_lib());
 
         ast.statements()
@@ -893,6 +893,7 @@ impl Engine {
                 EvalAltResult::Return(out, _) => Ok(out),
                 _ => Err(err),
             })
+            .map(|v| (v, state.operations))
     }
 
     /// Evaluate a file, but throw away the result and only return error (if any).
@@ -1015,10 +1016,11 @@ impl Engine {
             .get_function_by_signature(name, args.len(), true)
             .ok_or_else(|| Box::new(EvalAltResult::ErrorFunctionNotFound(name.into(), pos)))?;
 
-        let state = State::new(fn_lib);
+        let mut state = State::new(fn_lib);
+        let args = args.as_mut();
 
-        let result =
-            self.call_script_fn(Some(scope), &state, name, fn_def, args.as_mut(), pos, 0)?;
+        let (result, _) =
+            self.call_script_fn(Some(scope), &mut state, name, fn_def, args, pos, 0, 0)?;
 
         let return_type = self.map_type_name(result.type_name());
 
@@ -1058,6 +1060,84 @@ impl Engine {
         optimize_into_ast(self, scope, stmt, fn_lib, optimization_level)
     }
 
+    /// Register a callback for script evaluation progress.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
+    /// # use std::sync::RwLock;
+    /// # use std::sync::Arc;
+    /// use rhai::Engine;
+    ///
+    /// let result = Arc::new(RwLock::new(0_u64));
+    /// let logger = result.clone();
+    ///
+    /// let mut engine = Engine::new();
+    ///
+    /// engine.on_progress(move |ops| {
+    ///     if ops > 10000 {
+    ///         false
+    ///     } else if ops % 800 == 0 {
+    ///         *logger.write().unwrap() = ops;
+    ///         true
+    ///     } else {
+    ///         true
+    ///     }
+    /// });
+    ///
+    /// engine.consume("for x in range(0, 50000) {}")
+    ///     .expect_err("should error");
+    ///
+    /// assert_eq!(*result.read().unwrap(), 9600);
+    ///
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(feature = "sync")]
+    pub fn on_progress(&mut self, callback: impl Fn(u64) -> bool + Send + Sync + 'static) {
+        self.progress = Some(Box::new(callback));
+    }
+
+    /// Register a callback for script evaluation progress.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
+    /// # use std::cell::Cell;
+    /// # use std::rc::Rc;
+    /// use rhai::Engine;
+    ///
+    /// let result = Rc::new(Cell::new(0_u64));
+    /// let logger = result.clone();
+    ///
+    /// let mut engine = Engine::new();
+    ///
+    /// engine.on_progress(move |ops| {
+    ///     if ops > 10000 {
+    ///         false
+    ///     } else if ops % 800 == 0 {
+    ///         logger.set(ops);
+    ///         true
+    ///     } else {
+    ///         true
+    ///     }
+    /// });
+    ///
+    /// engine.consume("for x in range(0, 50000) {}")
+    ///     .expect_err("should error");
+    ///
+    /// assert_eq!(result.get(), 9600);
+    ///
+    /// # Ok(())
+    /// # }
+    /// ```
+    #[cfg(not(feature = "sync"))]
+    pub fn on_progress(&mut self, callback: impl Fn(u64) -> bool + 'static) {
+        self.progress = Some(Box::new(callback));
+    }
+
     /// Override default action of `print` (print to stdout using `println!`)
     ///
     /// # Example
@@ -1092,21 +1172,21 @@ impl Engine {
     ///
     /// ```
     /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
-    /// # use std::sync::RwLock;
-    /// # use std::sync::Arc;
+    /// # use std::cell::RefCell;
+    /// # use std::rc::Rc;
     /// use rhai::Engine;
     ///
-    /// let result = Arc::new(RwLock::new(String::from("")));
+    /// let result = Rc::new(RefCell::new(String::from("")));
     ///
     /// let mut engine = Engine::new();
     ///
     /// // Override action of 'print' function
     /// let logger = result.clone();
-    /// engine.on_print(move |s| logger.write().unwrap().push_str(s));
+    /// engine.on_print(move |s| logger.borrow_mut().push_str(s));
     ///
     /// engine.consume("print(40 + 2);")?;
     ///
-    /// assert_eq!(*result.read().unwrap(), "42");
+    /// assert_eq!(*result.borrow(), "42");
     /// # Ok(())
     /// # }
     /// ```
@@ -1149,21 +1229,21 @@ impl Engine {
     ///
     /// ```
     /// # fn main() -> Result<(), Box<rhai::EvalAltResult>> {
-    /// # use std::sync::RwLock;
-    /// # use std::sync::Arc;
+    /// # use std::cell::RefCell;
+    /// # use std::rc::Rc;
     /// use rhai::Engine;
     ///
-    /// let result = Arc::new(RwLock::new(String::from("")));
+    /// let result = Rc::new(RefCell::new(String::from("")));
     ///
     /// let mut engine = Engine::new();
     ///
     /// // Override action of 'print' function
     /// let logger = result.clone();
-    /// engine.on_debug(move |s| logger.write().unwrap().push_str(s));
+    /// engine.on_debug(move |s| logger.borrow_mut().push_str(s));
     ///
     /// engine.consume(r#"debug("hello");"#)?;
     ///
-    /// assert_eq!(*result.read().unwrap(), r#""hello""#);
+    /// assert_eq!(*result.borrow(), r#""hello""#);
     /// # Ok(())
     /// # }
     /// ```
diff --git a/src/engine.rs b/src/engine.rs
index 7ef00d45..6f159487 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -3,7 +3,7 @@
 use crate::any::{Dynamic, Union};
 use crate::calc_fn_hash;
 use crate::error::ParseErrorType;
-use crate::fn_native::{FnCallArgs, NativeFunctionABI, PrintCallback};
+use crate::fn_native::{FnCallArgs, NativeFunctionABI, PrintCallback, ProgressCallback};
 use crate::module::Module;
 use crate::optimize::OptimizationLevel;
 use crate::packages::{CorePackage, Package, PackageLibrary, PackagesCollection, StandardPackage};
@@ -28,7 +28,7 @@ use crate::stdlib::{
     format,
     iter::{empty, once, repeat},
     mem,
-    num::NonZeroUsize,
+    num::{NonZeroU64, NonZeroUsize},
     ops::{Deref, DerefMut},
     rc::Rc,
     string::{String, ToString},
@@ -48,12 +48,17 @@ pub type Array = Vec<Dynamic>;
 #[cfg(not(feature = "no_object"))]
 pub type Map = HashMap<String, Dynamic>;
 
+#[cfg(not(feature = "unchecked"))]
 #[cfg(debug_assertions)]
 pub const MAX_CALL_STACK_DEPTH: usize = 28;
 
+#[cfg(not(feature = "unchecked"))]
 #[cfg(not(debug_assertions))]
 pub const MAX_CALL_STACK_DEPTH: usize = 256;
 
+#[cfg(feature = "unchecked")]
+pub const MAX_CALL_STACK_DEPTH: usize = usize::MAX;
+
 pub const KEYWORD_PRINT: &str = "print";
 pub const KEYWORD_DEBUG: &str = "debug";
 pub const KEYWORD_TYPE_OF: &str = "type_of";
@@ -146,6 +151,9 @@ pub struct State<'a> {
     /// Level of the current scope.  The global (root) level is zero, a new block (or function call)
     /// is one level higher, and so on.
     pub scope_level: usize,
+
+    /// Number of operations performed.
+    pub operations: u64,
 }
 
 impl<'a> State<'a> {
@@ -155,6 +163,7 @@ impl<'a> State<'a> {
             always_search: false,
             fn_lib,
             scope_level: 0,
+            operations: 0,
         }
     }
     /// Does a certain script-defined function exist in the `State`?
@@ -304,6 +313,8 @@ pub struct Engine {
     pub(crate) print: Box<PrintCallback>,
     /// Closure for implementing the `debug` command.
     pub(crate) debug: Box<PrintCallback>,
+    /// Closure for progress reporting.
+    pub(crate) progress: Option<Box<ProgressCallback>>,
 
     /// Optimize the AST after compilation.
     pub(crate) optimization_level: OptimizationLevel,
@@ -311,6 +322,8 @@ pub struct Engine {
     ///
     /// Defaults to 28 for debug builds and 256 for non-debug builds.
     pub(crate) max_call_stack_depth: usize,
+    /// Maximum number of operations to run.
+    pub(crate) max_operations: Option<NonZeroU64>,
 }
 
 impl Default for Engine {
@@ -333,6 +346,9 @@ impl Default for Engine {
             print: Box::new(default_print),
             debug: Box::new(default_print),
 
+            // progress callback
+            progress: None,
+
             // optimization level
             #[cfg(feature = "no_optimize")]
             optimization_level: OptimizationLevel::None,
@@ -346,6 +362,7 @@ impl Default for Engine {
             optimization_level: OptimizationLevel::Full,
 
             max_call_stack_depth: MAX_CALL_STACK_DEPTH,
+            max_operations: None,
         };
 
         #[cfg(feature = "no_stdlib")]
@@ -471,6 +488,7 @@ impl Engine {
             type_names: Default::default(),
             print: Box::new(|_| {}),
             debug: Box::new(|_| {}),
+            progress: None,
 
             #[cfg(feature = "no_optimize")]
             optimization_level: OptimizationLevel::None,
@@ -484,6 +502,7 @@ impl Engine {
             optimization_level: OptimizationLevel::Full,
 
             max_call_stack_depth: MAX_CALL_STACK_DEPTH,
+            max_operations: None,
         }
     }
 
@@ -515,10 +534,17 @@ impl Engine {
 
     /// Set the maximum levels of function calls allowed for a script in order to avoid
     /// infinite recursion and stack overflows.
+    #[cfg(not(feature = "unchecked"))]
     pub fn set_max_call_levels(&mut self, levels: usize) {
         self.max_call_stack_depth = levels
     }
 
+    /// Set the maximum number of operations allowed for a script to run to avoid
+    /// consuming too much resources (0 for unlimited).
+    #[cfg(not(feature = "unchecked"))]
+    pub fn set_max_operations(&mut self, operations: u64) {
+        self.max_operations = NonZeroU64::new(operations);
+    }
     /// Set the module resolution service used by the `Engine`.
     ///
     /// Not available under the `no_module` feature.
@@ -537,7 +563,7 @@ impl Engine {
     pub(crate) fn call_fn_raw(
         &self,
         scope: Option<&mut Scope>,
-        state: &State,
+        state: &mut State,
         fn_name: &str,
         hashes: (u64, u64),
         args: &mut FnCallArgs,
@@ -546,6 +572,8 @@ impl Engine {
         pos: Position,
         level: usize,
     ) -> Result<(Dynamic, bool), Box<EvalAltResult>> {
+        self.inc_operations(state, pos)?;
+
         // Check for stack overflow
         if level > self.max_call_stack_depth {
             return Err(Box::new(EvalAltResult::ErrorStackOverflow(pos)));
@@ -554,9 +582,12 @@ impl Engine {
         // First search in script-defined functions (can override built-in)
         if hashes.1 > 0 {
             if let Some(fn_def) = state.get_function(hashes.1) {
-                return self
-                    .call_script_fn(scope, state, fn_name, fn_def, args, pos, level)
-                    .map(|v| (v, false));
+                let ops = state.operations;
+                let (result, operations) =
+                    self.call_script_fn(scope, &state, fn_name, fn_def, args, pos, level, ops)?;
+                state.operations = operations;
+                self.inc_operations(state, pos)?;
+                return Ok((result, false));
             }
         }
 
@@ -677,14 +708,17 @@ impl Engine {
         args: &mut FnCallArgs,
         pos: Position,
         level: usize,
-    ) -> Result<Dynamic, Box<EvalAltResult>> {
+        operations: u64,
+    ) -> Result<(Dynamic, u64), Box<EvalAltResult>> {
         match scope {
             // Extern scope passed in which is not empty
             Some(scope) if scope.len() > 0 => {
                 let scope_len = scope.len();
-                let mut state = State::new(state.fn_lib);
+                let mut local_state = State::new(state.fn_lib);
 
-                state.scope_level += 1;
+                local_state.operations = operations;
+                self.inc_operations(&mut local_state, pos)?;
+                local_state.scope_level += 1;
 
                 // Put arguments into scope as variables
                 scope.extend(
@@ -696,14 +730,14 @@ impl Engine {
                             args.into_iter().map(|v| mem::take(*v)),
                         )
                         .map(|(name, value)| {
-                            let var_name = unsafe_cast_var_name(name.as_str(), &state);
+                            let var_name = unsafe_cast_var_name(name.as_str(), &local_state);
                             (var_name, ScopeEntryType::Normal, value)
                         }),
                 );
 
                 // Evaluate the function at one higher level of call depth
                 let result = self
-                    .eval_stmt(scope, &mut state, &fn_def.body, level + 1)
+                    .eval_stmt(scope, &mut local_state, &fn_def.body, level + 1)
                     .or_else(|err| match *err {
                         // Convert return statement to return value
                         EvalAltResult::Return(x, _) => Ok(x),
@@ -725,13 +759,16 @@ impl Engine {
                 // No need to reset `state.scope_level` because it is thrown away
                 scope.rewind(scope_len);
 
-                return result;
+                return result.map(|v| (v, local_state.operations));
             }
             // No new scope - create internal scope
             _ => {
                 let mut scope = Scope::new();
-                let mut state = State::new(state.fn_lib);
-                state.scope_level += 1;
+                let mut local_state = State::new(state.fn_lib);
+
+                local_state.operations = operations;
+                self.inc_operations(&mut local_state, pos)?;
+                local_state.scope_level += 1;
 
                 // Put arguments into scope as variables
                 scope.extend(
@@ -748,7 +785,7 @@ impl Engine {
                 // Evaluate the function at one higher level of call depth
                 // No need to reset `state.scope_level` because it is thrown away
                 return self
-                    .eval_stmt(&mut scope, &mut state, &fn_def.body, level + 1)
+                    .eval_stmt(&mut scope, &mut local_state, &fn_def.body, level + 1)
                     .or_else(|err| match *err {
                         // Convert return statement to return value
                         EvalAltResult::Return(x, _) => Ok(x),
@@ -764,7 +801,8 @@ impl Engine {
                             err,
                             pos,
                         ))),
-                    });
+                    })
+                    .map(|v| (v, local_state.operations));
             }
         }
     }
@@ -788,7 +826,7 @@ impl Engine {
     /// **DO NOT** reuse the argument values unless for the first `&mut` argument - all others are silently replaced by `()`!
     fn exec_fn_call(
         &self,
-        state: &State,
+        state: &mut State,
         fn_name: &str,
         hash_fn_def: u64,
         args: &mut FnCallArgs,
@@ -827,7 +865,7 @@ impl Engine {
     fn eval_script_expr(
         &self,
         scope: &mut Scope,
-        state: &State,
+        state: &mut State,
         script: &Dynamic,
         pos: Position,
     ) -> Result<Dynamic, Box<EvalAltResult>> {
@@ -854,14 +892,20 @@ impl Engine {
         let ast = AST::new(statements, state.fn_lib.clone());
 
         // Evaluate the AST
-        self.eval_ast_with_scope_raw(scope, &ast)
-            .map_err(|err| err.new_position(pos))
+        let (result, operations) = self
+            .eval_ast_with_scope_raw(scope, &ast)
+            .map_err(|err| err.new_position(pos))?;
+
+        state.operations += operations;
+        self.inc_operations(state, pos)?;
+
+        return Ok(result);
     }
 
     /// Chain-evaluate a dot/index chain.
     fn eval_dot_index_chain_helper(
         &self,
-        state: &State,
+        state: &mut State,
         mut target: Target,
         rhs: &Expr,
         idx_values: &mut StaticVec<Dynamic>,
@@ -1145,7 +1189,7 @@ impl Engine {
     /// Get the value at the indexed position of a base type
     fn get_indexed_mut<'a>(
         &self,
-        state: &State,
+        state: &mut State,
         val: &'a mut Dynamic,
         is_ref: bool,
         mut idx: Dynamic,
@@ -1153,6 +1197,8 @@ impl Engine {
         op_pos: Position,
         create: bool,
     ) -> Result<Target<'a>, Box<EvalAltResult>> {
+        self.inc_operations(state, op_pos)?;
+
         match val {
             #[cfg(not(feature = "no_index"))]
             Dynamic(Union::Array(arr)) => {
@@ -1234,6 +1280,8 @@ impl Engine {
         rhs: &Expr,
         level: usize,
     ) -> Result<Dynamic, Box<EvalAltResult>> {
+        self.inc_operations(state, rhs.position())?;
+
         let mut lhs_value = self.eval_expr(scope, state, lhs, level)?;
         let rhs_value = self.eval_expr(scope, state, rhs, level)?;
 
@@ -1288,6 +1336,8 @@ impl Engine {
         expr: &Expr,
         level: usize,
     ) -> Result<Dynamic, Box<EvalAltResult>> {
+        self.inc_operations(state, expr.position())?;
+
         match expr {
             Expr::IntegerConstant(x) => Ok(x.0.into()),
             #[cfg(not(feature = "no_float"))]
@@ -1461,9 +1511,16 @@ impl Engine {
 
                 // First search in script-defined functions (can override built-in)
                 if let Some(fn_def) = module.get_qualified_scripted_fn(*hash_fn_def) {
-                    self.call_script_fn(None, state, name, fn_def, args.as_mut(), *pos, level)
+                    let args = args.as_mut();
+                    let ops = state.operations;
+                    let (result, operations) =
+                        self.call_script_fn(None, state, name, fn_def, args, *pos, level, ops)?;
+                    state.operations = operations;
+                    self.inc_operations(state, *pos)?;
+                    Ok(result)
                 } else {
                     // Then search in Rust functions
+                    self.inc_operations(state, *pos)?;
 
                     // Rust functions are indexed in two steps:
                     // 1) Calculate a hash in a similar manner to script-defined functions,
@@ -1538,6 +1595,8 @@ impl Engine {
         stmt: &Stmt,
         level: usize,
     ) -> Result<Dynamic, Box<EvalAltResult>> {
+        self.inc_operations(state, stmt.position())?;
+
         match stmt {
             // No-op
             Stmt::Noop(_) => Ok(Default::default()),
@@ -1546,11 +1605,10 @@ impl Engine {
             Stmt::Expr(expr) => {
                 let result = self.eval_expr(scope, state, expr, level)?;
 
-                Ok(if let Expr::Assignment(_) = *expr.as_ref() {
+                Ok(match expr.as_ref() {
                     // If it is an assignment, erase the result at the root
-                    Default::default()
-                } else {
-                    result
+                    Expr::Assignment(_) => Default::default(),
+                    _ => result,
                 })
             }
 
@@ -1574,25 +1632,32 @@ impl Engine {
             }
 
             // If-else statement
-            Stmt::IfThenElse(x) => self
-                .eval_expr(scope, state, &x.0, level)?
-                .as_bool()
-                .map_err(|_| Box::new(EvalAltResult::ErrorLogicGuard(x.0.position())))
-                .and_then(|guard_val| {
-                    if guard_val {
-                        self.eval_stmt(scope, state, &x.1, level)
-                    } else if let Some(stmt) = &x.2 {
-                        self.eval_stmt(scope, state, stmt, level)
-                    } else {
-                        Ok(Default::default())
-                    }
-                }),
+            Stmt::IfThenElse(x) => {
+                let (expr, if_block, else_block) = x.as_ref();
+
+                self.eval_expr(scope, state, expr, level)?
+                    .as_bool()
+                    .map_err(|_| Box::new(EvalAltResult::ErrorLogicGuard(expr.position())))
+                    .and_then(|guard_val| {
+                        if guard_val {
+                            self.eval_stmt(scope, state, if_block, level)
+                        } else if let Some(stmt) = else_block {
+                            self.eval_stmt(scope, state, stmt, level)
+                        } else {
+                            Ok(Default::default())
+                        }
+                    })
+            }
 
             // While loop
             Stmt::While(x) => loop {
-                match self.eval_expr(scope, state, &x.0, level)?.as_bool() {
-                    Ok(true) => match self.eval_stmt(scope, state, &x.1, level) {
-                        Ok(_) => (),
+                let (expr, body) = x.as_ref();
+
+                match self.eval_expr(scope, state, expr, level)?.as_bool() {
+                    Ok(true) => match self.eval_stmt(scope, state, body, level) {
+                        Ok(_) => {
+                            self.inc_operations(state, body.position())?;
+                        }
                         Err(err) => match *err {
                             EvalAltResult::ErrorLoopBreak(false, _) => (),
                             EvalAltResult::ErrorLoopBreak(true, _) => return Ok(Default::default()),
@@ -1600,14 +1665,18 @@ impl Engine {
                         },
                     },
                     Ok(false) => return Ok(Default::default()),
-                    Err(_) => return Err(Box::new(EvalAltResult::ErrorLogicGuard(x.0.position()))),
+                    Err(_) => {
+                        return Err(Box::new(EvalAltResult::ErrorLogicGuard(expr.position())))
+                    }
                 }
             },
 
             // Loop statement
             Stmt::Loop(body) => loop {
                 match self.eval_stmt(scope, state, body, level) {
-                    Ok(_) => (),
+                    Ok(_) => {
+                        self.inc_operations(state, body.position())?;
+                    }
                     Err(err) => match *err {
                         EvalAltResult::ErrorLoopBreak(false, _) => (),
                         EvalAltResult::ErrorLoopBreak(true, _) => return Ok(Default::default()),
@@ -1618,7 +1687,8 @@ impl Engine {
 
             // For loop
             Stmt::For(x) => {
-                let iter_type = self.eval_expr(scope, state, &x.1, level)?;
+                let (name, expr, stmt) = x.as_ref();
+                let iter_type = self.eval_expr(scope, state, expr, level)?;
                 let tid = iter_type.type_id();
 
                 if let Some(iter_fn) = self
@@ -1627,15 +1697,16 @@ impl Engine {
                     .or_else(|| self.packages.get_iter(tid))
                 {
                     // Add the loop variable
-                    let var_name = unsafe_cast_var_name(&x.0, &state);
+                    let var_name = unsafe_cast_var_name(name, &state);
                     scope.push(var_name, ());
                     let index = scope.len() - 1;
                     state.scope_level += 1;
 
                     for loop_var in iter_fn(iter_type) {
                         *scope.get_mut(index).0 = loop_var;
+                        self.inc_operations(state, stmt.position())?;
 
-                        match self.eval_stmt(scope, state, &x.2, level) {
+                        match self.eval_stmt(scope, state, stmt, level) {
                             Ok(_) => (),
                             Err(err) => match *err {
                                 EvalAltResult::ErrorLoopBreak(false, _) => (),
@@ -1690,26 +1761,29 @@ impl Engine {
 
             // Let statement
             Stmt::Let(x) if x.1.is_some() => {
-                let ((var_name, _), expr) = x.as_ref();
+                let ((var_name, pos), expr) = x.as_ref();
                 let val = self.eval_expr(scope, state, expr.as_ref().unwrap(), level)?;
                 let var_name = unsafe_cast_var_name(var_name, &state);
                 scope.push_dynamic_value(var_name, ScopeEntryType::Normal, val, false);
+                self.inc_operations(state, *pos)?;
                 Ok(Default::default())
             }
 
             Stmt::Let(x) => {
-                let ((var_name, _), _) = x.as_ref();
+                let ((var_name, pos), _) = x.as_ref();
                 let var_name = unsafe_cast_var_name(var_name, &state);
                 scope.push(var_name, ());
+                self.inc_operations(state, *pos)?;
                 Ok(Default::default())
             }
 
             // Const statement
             Stmt::Const(x) if x.1.is_constant() => {
-                let ((var_name, _), expr) = x.as_ref();
+                let ((var_name, pos), expr) = x.as_ref();
                 let val = self.eval_expr(scope, state, &expr, level)?;
                 let var_name = unsafe_cast_var_name(var_name, &state);
                 scope.push_dynamic_value(var_name, ScopeEntryType::Constant, val, true);
+                self.inc_operations(state, *pos)?;
                 Ok(Default::default())
             }
 
@@ -1718,7 +1792,7 @@ impl Engine {
 
             // Import statement
             Stmt::Import(x) => {
-                let (expr, (name, _)) = x.as_ref();
+                let (expr, (name, pos)) = x.as_ref();
 
                 #[cfg(feature = "no_module")]
                 unreachable!();
@@ -1736,6 +1810,7 @@ impl Engine {
 
                             let mod_name = unsafe_cast_var_name(name, &state);
                             scope.push_module(mod_name, module);
+                            self.inc_operations(state, *pos)?;
                             Ok(Default::default())
                         } else {
                             Err(Box::new(EvalAltResult::ErrorModuleNotFound(
@@ -1776,6 +1851,31 @@ impl Engine {
         }
     }
 
+    /// Check if the number of operations stay within limit.
+    fn inc_operations(&self, state: &mut State, pos: Position) -> Result<(), Box<EvalAltResult>> {
+        state.operations += 1;
+
+        #[cfg(not(feature = "unchecked"))]
+        {
+            // Guard against too many operations
+            if let Some(max) = self.max_operations {
+                if state.operations > max.get() {
+                    return Err(Box::new(EvalAltResult::ErrorTooManyOperations(pos)));
+                }
+            }
+        }
+
+        // Report progress - only in steps
+        if let Some(progress) = self.progress.as_ref() {
+            if !progress(state.operations) {
+                // Terminate script if progress returns false
+                return Err(Box::new(EvalAltResult::ErrorTerminated(pos)));
+            }
+        }
+
+        Ok(())
+    }
+
     /// Map a type_name into a pretty-print name
     pub(crate) fn map_type_name<'a>(&'a self, name: &'a str) -> &'a str {
         self.type_names
diff --git a/src/fn_native.rs b/src/fn_native.rs
index a8daac5f..335f9919 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -20,6 +20,11 @@ pub type PrintCallback = dyn Fn(&str) + Send + Sync + 'static;
 #[cfg(not(feature = "sync"))]
 pub type PrintCallback = dyn Fn(&str) + 'static;
 
+#[cfg(feature = "sync")]
+pub type ProgressCallback = dyn Fn(u64) -> bool + Send + Sync + 'static;
+#[cfg(not(feature = "sync"))]
+pub type ProgressCallback = dyn Fn(u64) -> bool + 'static;
+
 // Define callback function types
 #[cfg(feature = "sync")]
 pub trait ObjectGetCallback<T, U>: Fn(&mut T) -> U + Send + Sync + 'static {}
diff --git a/src/result.rs b/src/result.rs
index c33c810b..455a67ce 100644
--- a/src/result.rs
+++ b/src/result.rs
@@ -79,8 +79,12 @@ pub enum EvalAltResult {
     ErrorDotExpr(String, Position),
     /// Arithmetic error encountered. Wrapped value is the error message.
     ErrorArithmetic(String, Position),
+    /// Number of operations over maximum limit.
+    ErrorTooManyOperations(Position),
     /// Call stack over maximum limit.
     ErrorStackOverflow(Position),
+    /// The script is prematurely terminated.
+    ErrorTerminated(Position),
     /// Run-time error encountered. Wrapped value is the error message.
     ErrorRuntime(String, Position),
 
@@ -137,7 +141,9 @@ impl EvalAltResult {
             Self::ErrorInExpr(_) => "Malformed 'in' expression",
             Self::ErrorDotExpr(_, _) => "Malformed dot expression",
             Self::ErrorArithmetic(_, _) => "Arithmetic error",
+            Self::ErrorTooManyOperations(_) => "Too many operations",
             Self::ErrorStackOverflow(_) => "Stack overflow",
+            Self::ErrorTerminated(_) => "Script terminated.",
             Self::ErrorRuntime(_, _) => "Runtime error",
             Self::ErrorLoopBreak(true, _) => "Break statement not inside a loop",
             Self::ErrorLoopBreak(false, _) => "Continue statement not inside a loop",
@@ -183,7 +189,9 @@ impl fmt::Display for EvalAltResult {
             | Self::ErrorAssignmentToUnknownLHS(pos)
             | Self::ErrorInExpr(pos)
             | Self::ErrorDotExpr(_, pos)
-            | Self::ErrorStackOverflow(pos) => write!(f, "{} ({})", desc, pos),
+            | Self::ErrorTooManyOperations(pos)
+            | Self::ErrorStackOverflow(pos)
+            | Self::ErrorTerminated(pos) => write!(f, "{} ({})", desc, pos),
 
             Self::ErrorRuntime(s, pos) => {
                 write!(f, "{} ({})", if s.is_empty() { desc } else { s }, pos)
@@ -299,7 +307,9 @@ impl EvalAltResult {
             | Self::ErrorInExpr(pos)
             | Self::ErrorDotExpr(_, pos)
             | Self::ErrorArithmetic(_, pos)
+            | Self::ErrorTooManyOperations(pos)
             | Self::ErrorStackOverflow(pos)
+            | Self::ErrorTerminated(pos)
             | Self::ErrorRuntime(_, pos)
             | Self::ErrorLoopBreak(_, pos)
             | Self::Return(_, pos) => *pos,
@@ -335,7 +345,9 @@ impl EvalAltResult {
             | Self::ErrorInExpr(pos)
             | Self::ErrorDotExpr(_, pos)
             | Self::ErrorArithmetic(_, pos)
+            | Self::ErrorTooManyOperations(pos)
             | Self::ErrorStackOverflow(pos)
+            | Self::ErrorTerminated(pos)
             | Self::ErrorRuntime(_, pos)
             | Self::ErrorLoopBreak(_, pos)
             | Self::Return(_, pos) => *pos = new_position,
diff --git a/tests/operations.rs b/tests/operations.rs
new file mode 100644
index 00000000..2637ef3b
--- /dev/null
+++ b/tests/operations.rs
@@ -0,0 +1,93 @@
+#![cfg(not(feature = "unchecked"))]
+use rhai::{Engine, EvalAltResult};
+
+#[test]
+fn test_max_operations() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+    engine.set_max_operations(500);
+
+    engine.on_progress(|count| {
+        if count % 100 == 0 {
+            println!("{}", count);
+        }
+        true
+    });
+
+    engine.eval::<()>("let x = 0; while x < 20 { x += 1; }")?;
+
+    assert!(matches!(
+        *engine
+            .eval::<()>("for x in range(0, 500) {}")
+            .expect_err("should error"),
+        EvalAltResult::ErrorTooManyOperations(_)
+    ));
+
+    engine.set_max_operations(0);
+
+    engine.eval::<()>("for x in range(0, 10000) {}")?;
+
+    Ok(())
+}
+
+#[test]
+fn test_max_operations_functions() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+    engine.set_max_operations(500);
+
+    engine.on_progress(|count| {
+        if count % 100 == 0 {
+            println!("{}", count);
+        }
+        true
+    });
+
+    engine.eval::<()>(
+        r#"
+                fn inc(x) { x + 1 }
+                let x = 0;
+                while x < 20 { x = inc(x); }
+            "#,
+    )?;
+
+    assert!(matches!(
+        *engine
+            .eval::<()>(
+                r#"
+                    fn inc(x) { x + 1 }
+                    let x = 0;
+                    while x < 1000 { x = inc(x); }
+        "#
+            )
+            .expect_err("should error"),
+        EvalAltResult::ErrorTooManyOperations(_)
+    ));
+
+    Ok(())
+}
+
+#[test]
+fn test_max_operations_eval() -> Result<(), Box<EvalAltResult>> {
+    let mut engine = Engine::new();
+    engine.set_max_operations(500);
+
+    engine.on_progress(|count| {
+        if count % 100 == 0 {
+            println!("{}", count);
+        }
+        true
+    });
+
+    assert!(matches!(
+        *engine
+            .eval::<()>(
+                r#"
+                    let script = "for x in range(0, 500) {}";
+                    eval(script);
+            "#
+            )
+            .expect_err("should error"),
+        EvalAltResult::ErrorTooManyOperations(_)
+    ));
+
+    Ok(())
+}
diff --git a/tests/stack.rs b/tests/stack.rs
index 1eeb4250..5ebb550b 100644
--- a/tests/stack.rs
+++ b/tests/stack.rs
@@ -15,6 +15,7 @@ fn test_stack_overflow() -> Result<(), Box<EvalAltResult>> {
         325
     );
 
+    #[cfg(not(feature = "unchecked"))]
     match engine.eval::<()>(
         r"
             fn foo(n) { if n == 0 { 0 } else { n + foo(n-1) } }