diff --git a/Cargo.toml b/Cargo.toml
index ec00a10c..85b57974 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -34,6 +34,7 @@ no_index = []       # no arrays and indexing
 no_object = []      # no custom objects
 no_function = []    # no script-defined functions
 no_capture = []     # no automatic read/write binding of anonymous function's local variables to it's external context
+no_shared = []      # no explicit shared variables in the script code
 no_module = []      # no modules
 internals = []      # expose internal data structures
 unicode-xid-ident = ["unicode-xid"]  # allow Unicode Standard Annex #31 for identifiers.
diff --git a/src/any.rs b/src/any.rs
index 9d28c768..a7983030 100644
--- a/src/any.rs
+++ b/src/any.rs
@@ -1,6 +1,6 @@
 //! Helper module which defines the `Any` trait to to allow dynamic value handling.
 
-use crate::fn_native::{FnPtr, SendSync};
+use crate::fn_native::{FnPtr, SendSync, SharedMut};
 use crate::parser::{ImmutableString, INT};
 use crate::r#unsafe::{unsafe_cast_box, unsafe_try_cast};
 
@@ -18,8 +18,15 @@ use crate::stdlib::{
     boxed::Box,
     fmt,
     string::String,
+    ops::{Deref, DerefMut}
 };
 
+#[cfg(not(feature = "sync"))]
+use crate::stdlib::{rc::Rc, cell::{RefCell, Ref, RefMut}};
+
+#[cfg(feature = "sync")]
+use crate::stdlib::sync::{Arc, RwLock, RwLockReadGuard, RwLockWriteGuard};
+
 #[cfg(not(feature = "no_object"))]
 use crate::stdlib::collections::HashMap;
 
@@ -144,6 +151,88 @@ pub enum Union {
     Map(Box<Map>),
     FnPtr(Box<FnPtr>),
     Variant(Box<Box<dyn Variant>>),
+    Shared(Box<SharedCell>),
+}
+
+/// Internal Shared Dynamic representation.
+///
+/// Created with `Dynamic::into_shared()`.
+#[derive(Clone)]
+pub struct SharedCell {
+    value_type_id: TypeId,
+    value_type_name: &'static str,
+    container: SharedMut<Dynamic>
+}
+
+/// Dynamic's underlying `Variant` read guard that supports `Deref` for Variant's
+/// reference reading.
+///
+/// This data structure provides transparent interoperability between normal
+/// `Dynamic` types and Shared Dynamic references.
+#[derive(Debug)]
+pub struct DynamicReadLock<'d, T: Variant + Clone>(DynamicReadLockInner<'d, T>);
+
+#[derive(Debug)]
+enum DynamicReadLockInner<'d, T: Variant + Clone> {
+    Reference(&'d T),
+    #[cfg(not(feature = "sync"))]
+    Guard(Ref<'d, Dynamic>),
+    #[cfg(feature = "sync")]
+    Guard(RwLockReadGuard<'d, Dynamic>),
+}
+
+impl<'d, T: Variant + Clone> Deref for DynamicReadLock<'d, T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        match &self.0 {
+            DynamicReadLockInner::Reference(reference) => reference.deref(),
+            // unwrapping is safe because all checks already done in it's constructor
+            DynamicReadLockInner::Guard(guard) => guard.downcast_ref().unwrap(),
+        }
+    }
+}
+
+/// Dynamic's underlying `Variant` write guard that supports `Deref` and `DerefMut`
+/// for Variant's reference reading/writing.
+///
+/// This data structure provides transparent interoperability between normal
+/// `Dynamic` types and Shared Dynamic references.
+#[derive(Debug)]
+pub struct DynamicWriteLock<'d, T: Variant + Clone>(DynamicWriteLockInner<'d, T>);
+
+#[derive(Debug)]
+enum DynamicWriteLockInner<'d, T: Variant + Clone> {
+    Reference(&'d mut T),
+    #[cfg(not(feature = "sync"))]
+    Guard(RefMut<'d, Dynamic>),
+    #[cfg(feature = "sync")]
+    Guard(RwLockWriteGuard<'d, Dynamic>),
+}
+
+impl<'d, T: Variant + Clone> Deref for DynamicWriteLock<'d, T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &Self::Target {
+        match &self.0 {
+            DynamicWriteLockInner::Reference(reference) => reference.deref(),
+            // unwrapping is safe because all checks already done in it's constructor
+            DynamicWriteLockInner::Guard(guard) => guard.downcast_ref().unwrap(),
+        }
+    }
+}
+
+impl<'d, T: Variant + Clone> DerefMut for DynamicWriteLock<'d, T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        match &mut self.0 {
+            DynamicWriteLockInner::Reference(reference) => reference.deref_mut(),
+            // unwrapping is safe because all checks already done in it's constructor
+            DynamicWriteLockInner::Guard(guard) => guard.downcast_mut().unwrap(),
+        }
+    }
 }
 
 impl Dynamic {
@@ -156,7 +245,19 @@ impl Dynamic {
         }
     }
 
+    /// Does this `Dynamic` hold a shared data type
+    /// instead of one of the support system primitive types?
+    pub fn is_shared(&self) -> bool {
+        match self.0 {
+            Union::Shared(_) => true,
+            _ => false,
+        }
+    }
+
     /// Is the value held by this `Dynamic` a particular type?
+    ///
+    /// If the `Dynamic` is a Shared variant checking is performed on
+    /// top of it's internal value.
     pub fn is<T: Variant + Clone>(&self) -> bool {
         self.type_id() == TypeId::of::<T>()
             || match self.0 {
@@ -181,6 +282,7 @@ impl Dynamic {
             Union::Map(_) => TypeId::of::<Map>(),
             Union::FnPtr(_) => TypeId::of::<FnPtr>(),
             Union::Variant(value) => (***value).type_id(),
+            Union::Shared(cell) => (**cell).value_type_id,
         }
     }
 
@@ -203,6 +305,7 @@ impl Dynamic {
             #[cfg(not(feature = "no_std"))]
             Union::Variant(value) if value.is::<Instant>() => "timestamp",
             Union::Variant(value) => (***value).type_name(),
+            Union::Shared(cell) => (**cell).value_type_name,
         }
     }
 }
@@ -258,6 +361,7 @@ impl fmt::Display for Dynamic {
             #[cfg(not(feature = "no_std"))]
             Union::Variant(value) if value.is::<Instant>() => write!(f, "<timestamp>"),
             Union::Variant(value) => write!(f, "{}", (*value).type_name()),
+            Union::Shared(cell) => write!(f, "{}", (**cell).value_type_name),
         }
     }
 }
@@ -284,6 +388,7 @@ impl fmt::Debug for Dynamic {
             #[cfg(not(feature = "no_std"))]
             Union::Variant(value) if value.is::<Instant>() => write!(f, "<timestamp>"),
             Union::Variant(value) => write!(f, "{}", (*value).type_name()),
+            Union::Shared(cell) => write!(f, "{}", (**cell).value_type_name),
         }
     }
 }
@@ -304,6 +409,7 @@ impl Clone for Dynamic {
             Union::Map(ref value) => Self(Union::Map(value.clone())),
             Union::FnPtr(ref value) => Self(Union::FnPtr(value.clone())),
             Union::Variant(ref value) => (***value).clone_into_dynamic(),
+            Union::Shared(ref cell) => Self(Union::Shared(Box::new((**cell).clone())))
         }
     }
 }
@@ -415,10 +521,53 @@ impl Dynamic {
         Self(Union::Variant(Box::new(boxed)))
     }
 
+    /// Turns Dynamic into Shared Dynamic variant backed by runtime
+    /// mutable reference-counter container(`Arc<RwLock<Dynamic>>` or
+    /// `Rc<RefCell<Dynamic>` depending on `sync` feature).
+    ///
+    /// Instances of Shared Dynamic are relatively cheap to clone. All clones of
+    /// Shared Dynamic references the same chunk of memory.
+    ///
+    /// The Dynamic is capable to work transparently with the it's inner
+    /// data, seamlessly casting between ordinary instances of Dynamic and
+    /// Shared Dynamic instances.
+    ///
+    /// If original value already a Shared variant returns identity.
+    pub fn into_shared(self) -> Self {
+        match self.0 {
+            Union::Shared(..) => self,
+            _ => {
+                let cell = SharedCell {
+                    value_type_id: self.type_id(),
+                    value_type_name: self.type_name(),
+                    #[cfg(not(feature = "sync"))]
+                    container: Rc::new(RefCell::new(self)),
+                    #[cfg(feature = "sync")]
+                    container: Arc::new(RwLock::new(self)),
+                };
+
+                Self(Union::Shared(Box::new(cell)))
+            },
+        }
+    }
+
     /// Get a copy of the `Dynamic` value as a specific type.
     /// Casting to a `Dynamic` just returns as is.
     ///
-    /// Returns an error with the name of the value's actual type when the cast fails.
+    /// Returns None if types mismatched.
+    ///
+    /// # Shared Dynamic
+    ///
+    /// When accessing Shared Dynamic in sync mode(`sync` feature enabled)
+    /// can block current thread while the underlined data is being written.
+    ///
+    /// When accessing Shared Dynamic in NON-sync mode can **panic** if the data
+    /// is currently borrowed for write.
+    ///
+    /// ## Safety
+    ///
+    /// Both situations normally shouldn't happen since all operations in Rhai
+    /// use pass-by-value data and the script executed in a single thread.
     ///
     /// # Example
     ///
@@ -433,6 +582,24 @@ impl Dynamic {
     pub fn try_cast<T: Variant>(self) -> Option<T> {
         let type_id = TypeId::of::<T>();
 
+        if type_id == TypeId::of::<Dynamic>() {
+            return unsafe_cast_box::<_, T>(Box::new(self)).ok().map(|v| *v);
+        }
+
+        #[cfg(not(feature = "sync"))]
+        if let Union::Shared(cell) = self.0 {
+            let reference = cell.container.borrow();
+
+            return (*reference).clone().try_cast()
+        }
+
+        #[cfg(feature = "sync")]
+        if let Union::Shared(cell) = self.0 {
+            let read_lock = cell.container.read().unwrap();
+
+            return (*read_lock).clone().try_cast()
+        }
+
         if type_id == TypeId::of::<INT>() {
             return match self.0 {
                 Union::Int(value) => unsafe_try_cast(value),
@@ -496,9 +663,6 @@ impl Dynamic {
                 _ => None,
             };
         }
-        if type_id == TypeId::of::<Dynamic>() {
-            return unsafe_cast_box::<_, T>(Box::new(self)).ok().map(|v| *v);
-        }
 
         match self.0 {
             Union::Variant(value) => (*value).as_box_any().downcast().map(|x| *x).ok(),
@@ -511,7 +675,14 @@ impl Dynamic {
     ///
     /// # Panics
     ///
-    /// Panics if the cast fails (e.g. the type of the actual value is not the same as the specified type).
+    /// Panics if the cast fails (e.g. the type of the actual value is not the
+    /// same as the specified type), or if the data held by Shared Dynamic is
+    /// currently borrowed(when the `sync` feature disabled).
+    ///
+    /// # Notes
+    ///
+    /// If the `sync` feature enabled Shared Dynamic can block current thread
+    /// while the data being written.
     ///
     /// # Example
     ///
@@ -531,7 +702,104 @@ impl Dynamic {
     /// Casting to `Dynamic` just returns a reference to it.
     /// Returns `None` if the cast fails.
     #[inline(always)]
-    pub fn downcast_ref<T: Variant + Clone>(&self) -> Option<&T> {
+    pub fn read_lock<T: Variant + Clone>(&self) -> Option<DynamicReadLock<T>> {
+        match self.0 {
+            Union::Shared(ref cell) => {
+                let type_id = TypeId::of::<T>();
+
+                if type_id != TypeId::of::<Dynamic>() && cell.value_type_id != type_id {
+                    return None
+                }
+
+                #[cfg(not(feature = "sync"))]
+                return Some(DynamicReadLock(DynamicReadLockInner::Guard(
+                    cell.container.borrow()
+                )));
+
+                #[cfg(feature = "sync")]
+                return Some(DynamicReadLock(DynamicReadLockInner::Guard(
+                    cell.container.read().unwrap()
+                )));
+            },
+            _ => {
+                self.downcast_ref().map(|reference| {
+                    DynamicReadLock(DynamicReadLockInner::Reference(reference))
+                })
+            }
+        }
+    }
+
+    /// Get a copy of a specific type to the `Dynamic`.
+    /// Casting to `Dynamic` just returns a reference to it.
+    /// Returns `None` if the cast fails.
+    #[inline(always)]
+    pub fn read<T: Variant + Clone>(&self) -> Option<T> {
+        match self.0 {
+            Union::Shared(ref cell) => {
+                let type_id = TypeId::of::<T>();
+
+                if type_id != TypeId::of::<Dynamic>() && cell.value_type_id != type_id {
+                    return None
+                }
+
+                #[cfg(not(feature = "sync"))]
+                return Some(cell
+                    .container
+                    .borrow()
+                    .deref()
+                    .downcast_ref::<T>()
+                    .unwrap()
+                    .clone());
+
+                #[cfg(feature = "sync")]
+                return Some(cell
+                    .container
+                    .read()
+                    .unwrap()
+                    .deref()
+                    .downcast_ref::<T>()
+                    .unwrap()
+                    .clone());
+            },
+            _ => {
+                self.downcast_ref().cloned()
+            }
+        }
+    }
+
+    /// Get a mutable reference of a specific type to the `Dynamic`.
+    /// Casting to `Dynamic` just returns a mutable reference to it.
+    /// Returns `None` if the cast fails.
+    #[inline(always)]
+    pub fn write_lock<T: Variant + Clone>(&mut self) -> Option<DynamicWriteLock<T>> {
+        match self.0 {
+            Union::Shared(ref cell) => {
+                let type_id = TypeId::of::<T>();
+
+                if type_id != TypeId::of::<Dynamic>() && cell.value_type_id != type_id {
+                    return None
+                }
+
+                #[cfg(not(feature = "sync"))]
+                return Some(DynamicWriteLock(DynamicWriteLockInner::Guard(
+                    cell.container.borrow_mut()
+                )));
+
+                #[cfg(feature = "sync")]
+                return Some(DynamicWriteLock(DynamicWriteLockInner::Guard(
+                    cell.container.write().unwrap()
+                )));
+            },
+            _ => {
+                self.downcast_mut().map(|reference| {
+                    DynamicWriteLock(DynamicWriteLockInner::Reference(reference))
+                })
+            }
+        }
+    }
+
+    #[inline(always)]
+    fn downcast_ref<T: Variant + Clone>(&self) -> Option<&T> {
         let type_id = TypeId::of::<T>();
 
         if type_id == TypeId::of::<INT>() {
@@ -607,11 +875,8 @@ impl Dynamic {
         }
     }
 
-    /// Get a mutable reference of a specific type to the `Dynamic`.
-    /// Casting to `Dynamic` just returns a mutable reference to it.
-    /// Returns `None` if the cast fails.
     #[inline(always)]
-    pub fn downcast_mut<T: Variant + Clone>(&mut self) -> Option<&mut T> {
+    fn downcast_mut<T: Variant + Clone>(&mut self) -> Option<&mut T> {
         let type_id = TypeId::of::<T>();
 
         if type_id == TypeId::of::<INT>() {
diff --git a/src/engine.rs b/src/engine.rs
index 7806382f..06ff8d7f 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -91,6 +91,7 @@ pub const KEYWORD_EVAL: &str = "eval";
 pub const KEYWORD_FN_PTR: &str = "Fn";
 pub const KEYWORD_FN_PTR_CALL: &str = "call";
 pub const KEYWORD_FN_PTR_CURRY: &str = "curry";
+pub const KEYWORD_SHARED: &str = "shared";
 pub const KEYWORD_THIS: &str = "this";
 pub const FN_TO_STRING: &str = "to_string";
 #[cfg(not(feature = "no_object"))]
@@ -1068,6 +1069,18 @@ impl Engine {
 
         let val = target.as_mut();
 
+        // if val.is_shared() {
+        //     return self.get_indexed_mut(
+        //         state,
+        //         lib,
+        //         &mut Target::Value(val.read::<Dynamic>().unwrap()),
+        //         idx,
+        //         idx_pos,
+        //         create,
+        //         level,
+        //     );
+        // }
+
         match val {
             #[cfg(not(feature = "no_index"))]
             Dynamic(Union::Array(arr)) => {
@@ -1102,7 +1115,7 @@ impl Engine {
                     map.entry(index).or_insert(Default::default()).into()
                 } else {
                     let index = _idx
-                        .downcast_ref::<ImmutableString>()
+                        .read_lock::<ImmutableString>()
                         .ok_or_else(|| EvalAltResult::ErrorStringIndexExpr(idx_pos))?;
 
                     map.get_mut(index.as_str())
@@ -1280,7 +1293,11 @@ impl Engine {
                     )),
                     // Normal assignment
                     ScopeEntryType::Normal if op.is_empty() => {
-                        *lhs_ptr = rhs_val;
+                        if lhs_ptr.is_shared() {
+                            *lhs_ptr.write_lock::<Dynamic>().unwrap() = rhs_val;
+                        } else {
+                            *lhs_ptr = rhs_val;
+                        }
                         Ok(Default::default())
                     }
                     // Op-assignment - in order of precedence:
@@ -1298,8 +1315,13 @@ impl Engine {
                             .get_fn(hash_fn, false)
                             .or_else(|| self.packages.get_fn(hash_fn, false))
                         {
-                            // Overriding exact implementation
-                            func(self, lib, &mut [lhs_ptr, &mut rhs_val])?;
+                            if lhs_ptr.is_shared() {
+                                // Overriding exact implementation
+                                func(self, lib, &mut [&mut lhs_ptr.write_lock::<Dynamic>().unwrap(), &mut rhs_val])?;
+                            } else {
+                                // Overriding exact implementation
+                                func(self, lib, &mut [lhs_ptr, &mut rhs_val])?;
+                            }
                         } else if run_builtin_op_assignment(op, lhs_ptr, &rhs_val)?.is_none() {
                             // Not built in, map to `var = var op rhs`
                             let op = &op[..op.len() - 1]; // extract operator without =
@@ -1313,8 +1335,13 @@ impl Engine {
                                     level,
                                 )
                                 .map_err(|err| err.new_position(*op_pos))?;
-                            // Set value to LHS
-                            *lhs_ptr = value;
+                            if lhs_ptr.is_shared() {
+                                // Set value to LHS
+                                *lhs_ptr.write_lock::<Dynamic>().unwrap() = value;
+                            } else {
+                                // Set value to LHS
+                                *lhs_ptr = value;
+                            }
                         }
                         Ok(Default::default())
                     }
diff --git a/src/fn_call.rs b/src/fn_call.rs
index 162410f3..6ca46046 100644
--- a/src/fn_call.rs
+++ b/src/fn_call.rs
@@ -5,7 +5,7 @@ use crate::calc_fn_hash;
 use crate::engine::{
     search_imports, search_namespace, search_scope_only, Engine, Imports, State, KEYWORD_DEBUG,
     KEYWORD_EVAL, KEYWORD_FN_PTR, KEYWORD_FN_PTR_CALL, KEYWORD_FN_PTR_CURRY, KEYWORD_PRINT,
-    KEYWORD_TYPE_OF,
+    KEYWORD_TYPE_OF, KEYWORD_SHARED
 };
 use crate::error::ParseErrorType;
 use crate::fn_native::{FnCallArgs, FnPtr};
@@ -16,6 +16,7 @@ use crate::result::EvalAltResult;
 use crate::scope::Scope;
 use crate::token::Position;
 use crate::utils::StaticVec;
+use crate::stdlib::ops::Deref;
 
 #[cfg(not(feature = "no_function"))]
 use crate::{
@@ -540,7 +541,7 @@ impl Engine {
 
         let (result, updated) = if _fn_name == KEYWORD_FN_PTR_CALL && obj.is::<FnPtr>() {
             // FnPtr call
-            let fn_ptr = obj.downcast_ref::<FnPtr>().unwrap();
+            let fn_ptr = obj.read_lock::<FnPtr>().unwrap();
             let mut curry = fn_ptr.curry().iter().cloned().collect::<StaticVec<_>>();
             // Redirect function name
             let fn_name = fn_ptr.fn_name();
@@ -578,7 +579,7 @@ impl Engine {
             )
         } else if _fn_name == KEYWORD_FN_PTR_CURRY && obj.is::<FnPtr>() {
             // Curry call
-            let fn_ptr = obj.downcast_ref::<FnPtr>().unwrap();
+            let fn_ptr = obj.read_lock::<FnPtr>().unwrap();
             Ok((
                 FnPtr::new_unchecked(
                     fn_ptr.get_fn_name().clone(),
@@ -599,9 +600,9 @@ impl Engine {
 
             // Check if it is a map method call in OOP style
             #[cfg(not(feature = "no_object"))]
-            if let Some(map) = obj.downcast_ref::<Map>() {
+            if let Some(map) = obj.read_lock::<Map>() {
                 if let Some(val) = map.get(_fn_name) {
-                    if let Some(f) = val.downcast_ref::<FnPtr>() {
+                    if let Some(f) = val.read_lock::<FnPtr>() {
                         // Remap the function name
                         redirected = f.get_fn_name().clone();
                         _fn_name = &redirected;
@@ -697,6 +698,15 @@ impl Engine {
             .into());
         }
 
+        // Handle shared()
+        #[cfg(not(feature = "no_shared"))]
+        if name == KEYWORD_SHARED && args_expr.len() == 1 {
+            let expr = args_expr.get(0).unwrap();
+            let value = self.eval_expr(scope, mods, state, lib, this_ptr, expr, level)?;
+
+            return Ok(value.into_shared());
+        }
+
         // Handle eval()
         if name == KEYWORD_EVAL && args_expr.len() == 1 {
             let hash_fn = calc_fn_hash(empty(), name, 1, once(TypeId::of::<ImmutableString>()));
@@ -984,8 +994,8 @@ pub fn run_builtin_binary_op(
             _ => (),
         }
     } else if args_type == TypeId::of::<ImmutableString>() {
-        let x = x.downcast_ref::<ImmutableString>().unwrap();
-        let y = y.downcast_ref::<ImmutableString>().unwrap();
+        let x = &*x.read_lock::<ImmutableString>().unwrap();
+        let y = &*y.read_lock::<ImmutableString>().unwrap();
 
         match op {
             "+" => return Ok(Some((x + y).into())),
@@ -1058,7 +1068,7 @@ pub fn run_builtin_op_assignment(
     }
 
     if args_type == TypeId::of::<INT>() {
-        let x = x.downcast_mut::<INT>().unwrap();
+        let mut x = x.write_lock::<INT>().unwrap();
         let y = y.clone().cast::<INT>();
 
         #[cfg(not(feature = "unchecked"))]
@@ -1094,7 +1104,7 @@ pub fn run_builtin_op_assignment(
             _ => (),
         }
     } else if args_type == TypeId::of::<bool>() {
-        let x = x.downcast_mut::<bool>().unwrap();
+        let mut x = x.write_lock::<bool>().unwrap();
         let y = y.clone().cast::<bool>();
 
         match op {
@@ -1103,18 +1113,18 @@ pub fn run_builtin_op_assignment(
             _ => (),
         }
     } else if args_type == TypeId::of::<ImmutableString>() {
-        let x = x.downcast_mut::<ImmutableString>().unwrap();
-        let y = y.downcast_ref::<ImmutableString>().unwrap();
+        let mut x = x.write_lock::<ImmutableString>().unwrap();
+        let y = y.read_lock::<ImmutableString>().unwrap();
 
         match op {
-            "+=" => return Ok(Some(*x += y)),
+            "+=" => return Ok(Some(*x += y.deref())),
             _ => (),
         }
     }
 
     #[cfg(not(feature = "no_float"))]
     if args_type == TypeId::of::<FLOAT>() {
-        let x = x.downcast_mut::<FLOAT>().unwrap();
+        let mut x = x.write_lock::<FLOAT>().unwrap();
         let y = y.clone().cast::<FLOAT>();
 
         match op {
diff --git a/src/fn_native.rs b/src/fn_native.rs
index 5751351c..e51bf71b 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -1,6 +1,6 @@
 //! Module defining interfaces to native-Rust functions.
 
-use crate::any::Dynamic;
+use crate::any::{Dynamic, Variant};
 use crate::calc_fn_hash;
 use crate::engine::Engine;
 use crate::module::Module;
@@ -18,9 +18,9 @@ use crate::stdlib::{boxed::Box, convert::TryFrom, fmt, iter::empty, string::Stri
 use crate::stdlib::mem;
 
 #[cfg(not(feature = "sync"))]
-use crate::stdlib::rc::Rc;
+use crate::stdlib::{rc::Rc, cell::{RefCell, Ref, RefMut}};
 #[cfg(feature = "sync")]
-use crate::stdlib::sync::Arc;
+use crate::stdlib::sync::{Arc, RwLock};
 
 /// Trait that maps to `Send + Sync` only under the `sync` feature.
 #[cfg(feature = "sync")]
@@ -34,11 +34,18 @@ pub trait SendSync {}
 #[cfg(not(feature = "sync"))]
 impl<T> SendSync for T {}
 
+/// Immutable reference counting container
 #[cfg(not(feature = "sync"))]
 pub type Shared<T> = Rc<T>;
 #[cfg(feature = "sync")]
 pub type Shared<T> = Arc<T>;
 
+/// Mutable reference counting container(read-write lock)
+#[cfg(not(feature = "sync"))]
+pub type SharedMut<T> = Rc<RefCell<T>>;
+#[cfg(feature = "sync")]
+pub type SharedMut<T> = Arc<RwLock<T>>;
+
 /// Consume a `Shared` resource and return a mutable reference to the wrapped value.
 /// If the resource is shared (i.e. has other outstanding references), a cloned copy is used.
 pub fn shared_make_mut<T: Clone>(value: &mut Shared<T>) -> &mut T {
diff --git a/src/fn_register.rs b/src/fn_register.rs
index c2eb5534..ddcddff3 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -2,7 +2,7 @@
 
 #![allow(non_snake_case)]
 
-use crate::any::{Dynamic, Variant};
+use crate::any::{Dynamic, Variant, DynamicWriteLock};
 use crate::engine::Engine;
 use crate::fn_native::{CallableFunction, FnAny, FnCallArgs, SendSync};
 use crate::module::Module;
@@ -97,11 +97,11 @@ pub trait RegisterResultFn<FN, ARGS> {
 pub struct Mut<T>(T);
 //pub struct Ref<T>(T);
 
-/// Dereference into &mut.
+/// Dereference into DynamicWriteLock
 #[inline(always)]
-pub fn by_ref<T: Variant + Clone>(data: &mut Dynamic) -> &mut T {
-    // Directly cast the &mut Dynamic into &mut T to access the underlying data.
-    data.downcast_mut::<T>().unwrap()
+pub fn by_ref<T: Variant + Clone>(data: &mut Dynamic) -> DynamicWriteLock<T> {
+    // Directly cast the &mut Dynamic into DynamicWriteLock to access the underlying data.
+    data.write_lock::<T>().unwrap()
 }
 
 /// Dereference into value.
@@ -124,24 +124,23 @@ pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
 
 /// This macro creates a closure wrapping a registered function.
 macro_rules! make_func {
-	($fn:ident : $map:expr ; $($par:ident => $convert:expr),*) => {
+	($fn:ident : $map:expr ; $($par:ident => $let:stmt => $convert:expr => $arg:expr),*) => {
 //   ^ function pointer
 //               ^ result mapping function
 //                           ^ function parameter generic type name (A, B, C etc.)
-//                                           ^ dereferencing function
+//                                          ^ argument let statement(e.g. let mut A ...)
+//                                                       ^ dereferencing function
+//                                                                         ^ argument reference expression(like A, *B, &mut C etc)
 
 		Box::new(move |_: &Engine, _: &Module, args: &mut FnCallArgs| {
             // The arguments are assumed to be of the correct number and types!
 
 			let mut _drain = args.iter_mut();
-			$(
-			// Downcast every element, panic in case of a type mismatch (which shouldn't happen).
-			// Call the user-supplied function using ($convert) to access it either by value or by reference.
-			let $par = ($convert)(_drain.next().unwrap());
-			)*
+			$($let)*
+			$($par = ($convert)(_drain.next().unwrap()); )*
 
             // Call the function with each parameter value
-			let r = $fn($($par),*);
+			let r = $fn($($arg),*);
 
             // Map the result
             $map(r)
@@ -181,12 +180,13 @@ macro_rules! def_register {
     () => {
         def_register!(imp from_pure :);
     };
-    (imp $abi:ident : $($par:ident => $mark:ty => $param:ty => $clone:expr),*) => {
+    (imp $abi:ident : $($par:ident => $arg:expr => $mark:ty => $param:ty => $let:stmt => $clone:expr),*) => {
     //   ^ function ABI type
     //                  ^ function parameter generic type name (A, B, C etc.)
-    //                                ^ function parameter marker type (T, Ref<T> or Mut<T>)
-    //                                            ^ function parameter actual type (T, &T or &mut T)
-    //                                                         ^ dereferencing function
+//                                    ^ call argument(like A, *B, &mut C etc)
+    //                                            ^ function parameter marker type (T, Ref<T> or Mut<T>)
+    //                                                         ^ function parameter actual type (T, &T or &mut T)
+    //                                                                      ^ argument let statement
         impl<
             $($par: Variant + Clone,)*
             FN: Fn($($param),*) -> RET + SendSync + 'static,
@@ -196,7 +196,7 @@ macro_rules! def_register {
             fn register_fn(&mut self, name: &str, f: FN) -> &mut Self {
                 self.global_module.set_fn(name, FnAccess::Public,
                     &[$(map_type_id::<$par>()),*],
-                    CallableFunction::$abi(make_func!(f : map_dynamic ; $($par => $clone),*))
+                    CallableFunction::$abi(make_func!(f : map_dynamic ; $($par => $let => $clone => $arg),*))
                 );
                 self
             }
@@ -210,7 +210,7 @@ macro_rules! def_register {
             fn register_result_fn(&mut self, name: &str, f: FN) -> &mut Self {
                 self.global_module.set_fn(name, FnAccess::Public,
                     &[$(map_type_id::<$par>()),*],
-                    CallableFunction::$abi(make_func!(f : map_result ; $($par => $clone),*))
+                    CallableFunction::$abi(make_func!(f : map_result ; $($par => $let => $clone => $arg),*))
                 );
                 self
             }
@@ -219,11 +219,11 @@ macro_rules! def_register {
         //def_register!(imp_pop $($par => $mark => $param),*);
     };
     ($p0:ident $(, $p:ident)*) => {
-        def_register!(imp from_pure   : $p0 => $p0      => $p0      => by_value $(, $p => $p => $p => by_value)*);
-        def_register!(imp from_method : $p0 => Mut<$p0> => &mut $p0 => by_ref   $(, $p => $p => $p => by_value)*);
+        def_register!(imp from_pure   : $p0 => $p0      => $p0      => $p0      => let $p0     => by_value $(, $p => $p => $p => $p => let $p => by_value)*);
+        def_register!(imp from_method : $p0 => &mut $p0  => Mut<$p0> => &mut $p0 => let mut $p0 => by_ref   $(, $p => $p => $p => $p => let $p => by_value)*);
         //                ^ CallableFunction
-        // handle the first parameter                                  ^ first parameter passed through
-        //                                                                                            ^ others passed by value (by_value)
+        // handle the first parameter                                              ^ first parameter passed through
+        //                                                                                                     ^ others passed by value (by_value)
 
         // Currently does not support first argument which is a reference, as there will be
         // conflicting implementations since &T: Any and T: Any cannot be distinguished
diff --git a/src/module.rs b/src/module.rs
index 8eacde7b..fa23abd6 100644
--- a/src/module.rs
+++ b/src/module.rs
@@ -442,7 +442,7 @@ impl Module {
     ///                     // Since it is a primary type, it can also be cheaply copied
     ///                     let double = args[1].clone().cast::<bool>();
     ///                     // Get a mutable reference to the first argument.
-    ///                     let x = args[0].downcast_mut::<i64>().unwrap();
+    ///                     let mut x = args[0].write_lock::<i64>().unwrap();
     ///
     ///                     let orig = *x;
     ///
@@ -534,7 +534,7 @@ impl Module {
         func: impl Fn(&mut A) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |_: &Engine, _: &Module, args: &mut FnCallArgs| {
-            func(args[0].downcast_mut::<A>().unwrap()).map(Dynamic::from)
+            func(&mut args[0].write_lock::<A>().unwrap()).map(Dynamic::from)
         };
         let arg_types = [TypeId::of::<A>()];
         self.set_fn(name, Public, &arg_types, Func::from_method(Box::new(f)))
@@ -615,9 +615,9 @@ impl Module {
     ) -> u64 {
         let f = move |_: &Engine, _: &Module, args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();
-            let a = args[0].downcast_mut::<A>().unwrap();
+            let mut a = args[0].write_lock::<A>().unwrap();
 
-            func(a, b).map(Dynamic::from)
+            func(&mut a, b).map(Dynamic::from)
         };
         let arg_types = [TypeId::of::<A>(), TypeId::of::<B>()];
         self.set_fn(name, Public, &arg_types, Func::from_method(Box::new(f)))
@@ -739,9 +739,9 @@ impl Module {
         let f = move |_: &Engine, _: &Module, args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();
             let c = mem::take(args[2]).cast::<C>();
-            let a = args[0].downcast_mut::<A>().unwrap();
+            let mut a = args[0].write_lock::<A>().unwrap();
 
-            func(a, b, c).map(Dynamic::from)
+            func(&mut a, b, c).map(Dynamic::from)
         };
         let arg_types = [TypeId::of::<A>(), TypeId::of::<B>(), TypeId::of::<C>()];
         self.set_fn(name, Public, &arg_types, Func::from_method(Box::new(f)))
@@ -773,9 +773,9 @@ impl Module {
         let f = move |_: &Engine, _: &Module, args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();
             let c = mem::take(args[2]).cast::<C>();
-            let a = args[0].downcast_mut::<A>().unwrap();
+            let mut a = args[0].write_lock::<A>().unwrap();
 
-            func(a, b, c).map(Dynamic::from)
+            func(&mut a, b, c).map(Dynamic::from)
         };
         let arg_types = [TypeId::of::<A>(), TypeId::of::<B>(), TypeId::of::<C>()];
         self.set_fn(
@@ -896,9 +896,9 @@ impl Module {
             let b = mem::take(args[1]).cast::<B>();
             let c = mem::take(args[2]).cast::<C>();
             let d = mem::take(args[3]).cast::<D>();
-            let a = args[0].downcast_mut::<A>().unwrap();
+            let mut a = args[0].write_lock::<A>().unwrap();
 
-            func(a, b, c, d).map(Dynamic::from)
+            func(&mut a, b, c, d).map(Dynamic::from)
         };
         let arg_types = [
             TypeId::of::<A>(),
diff --git a/src/packages/array_basic.rs b/src/packages/array_basic.rs
index c0bea7ee..5ca3b1c7 100644
--- a/src/packages/array_basic.rs
+++ b/src/packages/array_basic.rs
@@ -34,7 +34,7 @@ fn pad<T: Variant + Clone>(
     _: &Module,
     args: &mut [&mut Dynamic],
 ) -> FuncReturn<()> {
-    let len = *args[1].downcast_ref::<INT>().unwrap();
+    let len = *args[1].read_lock::<INT>().unwrap();
 
     // Check if array will be over max size limit
     #[cfg(not(feature = "unchecked"))]
@@ -52,7 +52,7 @@ fn pad<T: Variant + Clone>(
 
     if len > 0 {
         let item = args[2].clone();
-        let list = args[0].downcast_mut::<Array>().unwrap();
+        let mut list = args[0].write_lock::<Array>().unwrap();
 
         if len as usize > list.len() {
             list.resize(len as usize, item);
diff --git a/src/packages/string_more.rs b/src/packages/string_more.rs
index 6ddc2b0c..b50d6915 100644
--- a/src/packages/string_more.rs
+++ b/src/packages/string_more.rs
@@ -228,7 +228,7 @@ def_package!(crate:MoreStringPackage:"Additional string utilities, including str
         "pad",
         &[TypeId::of::<ImmutableString>(), TypeId::of::<INT>(), TypeId::of::<char>()],
         |_engine: &Engine, _: &Module, args: &mut [&mut Dynamic]| {
-            let len = *args[1].downcast_ref::< INT>().unwrap();
+            let len = *args[1].read_lock::< INT>().unwrap();
 
             // Check if string will be over max size limit
             #[cfg(not(feature = "unchecked"))]
@@ -243,7 +243,7 @@ def_package!(crate:MoreStringPackage:"Additional string utilities, including str
 
             if len > 0 {
                 let ch = mem::take(args[2]).cast::<char>();
-                let s = args[0].downcast_mut::<ImmutableString>().unwrap();
+                let mut s = args[0].write_lock::<ImmutableString>().unwrap();
 
                 let orig_len = s.chars().count();
 
diff --git a/src/scope.rs b/src/scope.rs
index 5693554c..b095f782 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -333,7 +333,7 @@ impl<'a> Scope<'a> {
             .iter()
             .rev()
             .find(|Entry { name: key, .. }| name == key)
-            .and_then(|Entry { value, .. }| value.downcast_ref::<T>().cloned())
+            .and_then(|Entry { value, .. }| value.read::<T>())
     }
 
     /// Update the value of the named entry.
diff --git a/src/token.rs b/src/token.rs
index 7d1a07ba..bd496afb 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -2,7 +2,7 @@
 
 use crate::engine::{
     Engine, KEYWORD_DEBUG, KEYWORD_EVAL, KEYWORD_FN_PTR, KEYWORD_FN_PTR_CALL, KEYWORD_FN_PTR_CURRY,
-    KEYWORD_PRINT, KEYWORD_THIS, KEYWORD_TYPE_OF,
+    KEYWORD_SHARED, KEYWORD_PRINT, KEYWORD_THIS, KEYWORD_TYPE_OF,
 };
 
 use crate::error::LexError;
@@ -507,7 +507,7 @@ impl Token {
             | "async" | "await" | "yield" => Reserved(syntax.into()),
 
             KEYWORD_PRINT | KEYWORD_DEBUG | KEYWORD_TYPE_OF | KEYWORD_EVAL | KEYWORD_FN_PTR
-            | KEYWORD_FN_PTR_CALL | KEYWORD_FN_PTR_CURRY | KEYWORD_THIS => Reserved(syntax.into()),
+            | KEYWORD_FN_PTR_CALL | KEYWORD_FN_PTR_CURRY | KEYWORD_SHARED | KEYWORD_THIS => Reserved(syntax.into()),
 
             _ => return None,
         })