diff --git a/reid/src/ast/mod.rs b/reid/src/ast/mod.rs
index 665f286..2811250 100644
--- a/reid/src/ast/mod.rs
+++ b/reid/src/ast/mod.rs
@@ -27,7 +27,7 @@ pub enum TypeKind {
     String,
     Array(Box<TypeKind>, u64),
     Custom(String),
-    Borrow(Box<TypeKind>),
+    Borrow(Box<TypeKind>, bool),
 }
 
 #[derive(Debug, Clone)]
@@ -43,7 +43,7 @@ pub struct Expression(pub ExpressionKind, pub TokenRange);
 #[derive(Debug, Clone)]
 pub enum ExpressionKind {
     VariableName(String),
-    Borrow(String),
+    Borrow(String, bool),
     Deref(String),
     Literal(Literal),
     Array(Vec<Expression>),
diff --git a/reid/src/ast/parse.rs b/reid/src/ast/parse.rs
index be2d6ab..47c1b59 100644
--- a/reid/src/ast/parse.rs
+++ b/reid/src/ast/parse.rs
@@ -26,8 +26,15 @@ impl Parse for Type {
             TypeKind::Array(Box::new(inner.0), length)
         } else if let Some(Token::Et) = stream.peek() {
             stream.expect(Token::Et)?;
+            let mutable = if let Some(Token::MutKeyword) = stream.peek() {
+                stream.next();
+                true
+            } else {
+                false
+            };
+
             let inner = stream.parse::<Type>()?;
-            TypeKind::Borrow(Box::new(inner.0))
+            TypeKind::Borrow(Box::new(inner.0), mutable)
         } else {
             if let Some(Token::Identifier(ident)) = stream.next() {
                 match &*ident {
@@ -87,12 +94,19 @@ impl Parse for PrimaryExpression {
                 Kind::StructExpression(stream.parse()?),
                 stream.get_range().unwrap(),
             )
+        } else if let (Some(Token::Et), Some(Token::MutKeyword)) = (stream.peek(), stream.peek2()) {
+            stream.next(); // Consume Et
+            stream.next(); // Consume mut
+            let Some(Token::Identifier(name)) = stream.next() else {
+                return Err(stream.expected_err("identifier")?);
+            };
+            Expression(Kind::Borrow(name, true), stream.get_range().unwrap())
         } else if let (Some(Token::Et), Some(Token::Identifier(name))) =
             (stream.peek(), stream.peek2())
         {
             stream.next(); // Consume Et
             stream.next(); // Consume identifier
-            Expression(Kind::Borrow(name), stream.get_range().unwrap())
+            Expression(Kind::Borrow(name, false), stream.get_range().unwrap())
         } else if let (Some(Token::Star), Some(Token::Identifier(name))) =
             (stream.peek(), stream.peek2())
         {
diff --git a/reid/src/ast/process.rs b/reid/src/ast/process.rs
index 3b872b5..395602a 100644
--- a/reid/src/ast/process.rs
+++ b/reid/src/ast/process.rs
@@ -225,11 +225,14 @@ impl ast::Expression {
                 mir::TypeKind::Vague(mir::VagueType::Unknown),
                 name.clone(),
             ),
-            ast::ExpressionKind::Borrow(name) => mir::ExprKind::Borrow(NamedVariableRef(
-                mir::TypeKind::Vague(mir::VagueType::Unknown),
-                name.clone(),
-                self.1.as_meta(module_id),
-            )),
+            ast::ExpressionKind::Borrow(name, mutable) => mir::ExprKind::Borrow(
+                NamedVariableRef(
+                    mir::TypeKind::Vague(mir::VagueType::Unknown),
+                    name.clone(),
+                    self.1.as_meta(module_id),
+                ),
+                *mutable,
+            ),
             ast::ExpressionKind::Deref(name) => mir::ExprKind::Deref(NamedVariableRef(
                 mir::TypeKind::Vague(mir::VagueType::Unknown),
                 name.clone(),
@@ -287,8 +290,8 @@ impl From<ast::TypeKind> for mir::TypeKind {
             }
             ast::TypeKind::String => mir::TypeKind::StringPtr,
             ast::TypeKind::Custom(name) => mir::TypeKind::CustomType(name.clone()),
-            ast::TypeKind::Borrow(type_kind) => {
-                mir::TypeKind::Borrow(Box::new(mir::TypeKind::from(*type_kind.clone())))
+            ast::TypeKind::Borrow(type_kind, mutable) => {
+                mir::TypeKind::Borrow(Box::new(mir::TypeKind::from(*type_kind.clone())), *mutable)
             }
         }
     }
diff --git a/reid/src/codegen.rs b/reid/src/codegen.rs
index 6343017..29d0bd1 100644
--- a/reid/src/codegen.rs
+++ b/reid/src/codegen.rs
@@ -114,6 +114,13 @@ pub enum StackValueKind {
 }
 
 impl StackValueKind {
+    fn mutable(mutable: bool, instr: InstructionValue) -> StackValueKind {
+        match mutable {
+            true => StackValueKind::Mutable(instr),
+            false => StackValueKind::Immutable(instr),
+        }
+    }
+
     fn instr(&self) -> InstructionValue {
         match &self {
             StackValueKind::Immutable(val) => *val,
@@ -934,13 +941,16 @@ impl mir::Expression {
                     TypeKind::CustomType(name.clone()),
                 ))
             }
-            mir::ExprKind::Borrow(varref) => {
+            mir::ExprKind::Borrow(varref, mutable) => {
                 varref.0.known().expect("variable type unknown");
                 let v = scope
                     .stack_values
                     .get(&varref.1)
                     .expect("Variable reference not found?!");
-                Some(v.clone())
+                Some(StackValue(
+                    StackValueKind::mutable(*mutable, v.0.instr()),
+                    v.1.clone(),
+                ))
             }
             mir::ExprKind::Deref(varref) => {
                 varref.0.known().expect("variable type unknown");
@@ -1166,7 +1176,7 @@ impl TypeKind {
             TypeKind::Ptr(type_kind) => {
                 Type::Ptr(Box::new(type_kind.get_type(type_vals, typedefs)))
             }
-            TypeKind::Borrow(type_kind) => {
+            TypeKind::Borrow(type_kind, _) => {
                 Type::Ptr(Box::new(type_kind.get_type(type_vals, typedefs)))
             }
         }
@@ -1213,7 +1223,7 @@ impl TypeKind {
                 ),
                 size_bits: self.size_of(),
             }),
-            TypeKind::Ptr(inner) | TypeKind::Borrow(inner) => {
+            TypeKind::Ptr(inner) | TypeKind::Borrow(inner, _) => {
                 DebugTypeData::Pointer(DebugPointerType {
                     name,
                     pointee: inner.get_debug_type_hard(
diff --git a/reid/src/mir/fmt.rs b/reid/src/mir/fmt.rs
index 0b4e2d7..7a9f2b3 100644
--- a/reid/src/mir/fmt.rs
+++ b/reid/src/mir/fmt.rs
@@ -208,7 +208,8 @@ impl Display for ExprKind {
                 write_access(f, name)?;
                 write!(f, "<{}>", type_kind)
             }
-            ExprKind::Borrow(var_ref) => write!(f, "&{}", var_ref),
+            ExprKind::Borrow(var_ref, false) => write!(f, "&{}", var_ref),
+            ExprKind::Borrow(var_ref, true) => write!(f, "&mut {}", var_ref),
             ExprKind::Deref(var_ref) => write!(f, "*{}", var_ref),
         }
     }
diff --git a/reid/src/mir/impl.rs b/reid/src/mir/impl.rs
index aeae147..8e22a07 100644
--- a/reid/src/mir/impl.rs
+++ b/reid/src/mir/impl.rs
@@ -46,7 +46,7 @@ impl TypeKind {
             TypeKind::CustomType(_) => 32,
             TypeKind::Ptr(_) => 64,
             TypeKind::Vague(_) => panic!("Tried to sizeof a vague type!"),
-            TypeKind::Borrow(_) => 64,
+            TypeKind::Borrow(_, _) => 64,
         }
     }
 
@@ -69,7 +69,14 @@ impl TypeKind {
             TypeKind::CustomType(_) => 32,
             TypeKind::Ptr(_) => 64,
             TypeKind::Vague(_) => panic!("Tried to sizeof a vague type!"),
-            TypeKind::Borrow(_) => 64,
+            TypeKind::Borrow(_, _) => 64,
+        }
+    }
+
+    pub fn is_mutable(&self) -> bool {
+        match self {
+            TypeKind::Borrow(_, true) => true,
+            _ => false,
         }
     }
 }
@@ -221,14 +228,14 @@ impl Expression {
             }
             Accessed(_, type_kind, _) => Ok((ReturnKind::Soft, type_kind.clone())),
             Struct(name, _) => Ok((ReturnKind::Soft, TypeKind::CustomType(name.clone()))),
-            Borrow(var) => {
+            Borrow(var, mutable) => {
                 let ret_type = var.return_type()?;
-                Ok((ret_type.0, TypeKind::Borrow(Box::new(ret_type.1))))
+                Ok((ret_type.0, TypeKind::Borrow(Box::new(ret_type.1), *mutable)))
             }
             Deref(var) => {
                 let (kind, ret_type) = var.return_type()?;
                 match ret_type.resolve_weak(refs) {
-                    TypeKind::Borrow(type_kind) => Ok((kind, *type_kind)),
+                    TypeKind::Borrow(type_kind, _) => Ok((kind, *type_kind)),
                     _ => Err(ReturnTypeOther::DerefNonBorrow(var.2)),
                 }
             }
@@ -240,7 +247,7 @@ impl Expression {
             ExprKind::Variable(var_ref) => Some(var_ref),
             ExprKind::Indexed(lhs, _, _) => lhs.backing_var(),
             ExprKind::Accessed(lhs, _, _) => lhs.backing_var(),
-            ExprKind::Borrow(var) => Some(var),
+            ExprKind::Borrow(var, _) => Some(var),
             ExprKind::Deref(var) => Some(var),
             ExprKind::Block(block) => block.backing_var(),
             ExprKind::Array(_) => None,
@@ -335,7 +342,9 @@ impl TypeKind {
         let resolved = self.resolve_weak(refs);
         match resolved {
             TypeKind::Array(t, len) => TypeKind::Array(Box::new(t.resolve_ref(refs)), len),
-            TypeKind::Borrow(inner) => TypeKind::Borrow(Box::new(inner.resolve_ref(refs))),
+            TypeKind::Borrow(inner, mutable) => {
+                TypeKind::Borrow(Box::new(inner.resolve_ref(refs)), mutable)
+            }
             _ => resolved,
         }
     }
diff --git a/reid/src/mir/mod.rs b/reid/src/mir/mod.rs
index 2eb8320..8ef140c 100644
--- a/reid/src/mir/mod.rs
+++ b/reid/src/mir/mod.rs
@@ -110,7 +110,7 @@ pub enum TypeKind {
     #[error("{0}")]
     CustomType(String),
     #[error("Borrow({0})")]
-    Borrow(Box<TypeKind>),
+    Borrow(Box<TypeKind>, bool),
     #[error("Ptr({0})")]
     Ptr(Box<TypeKind>),
     #[error(transparent)]
@@ -247,7 +247,7 @@ pub enum ExprKind {
     FunctionCall(FunctionCall),
     If(IfExpression),
     Block(Block),
-    Borrow(NamedVariableRef),
+    Borrow(NamedVariableRef, bool),
     Deref(NamedVariableRef),
 }
 
diff --git a/reid/src/mir/typecheck.rs b/reid/src/mir/typecheck.rs
index 1d5232e..d438205 100644
--- a/reid/src/mir/typecheck.rs
+++ b/reid/src/mir/typecheck.rs
@@ -629,7 +629,7 @@ impl Expression {
                 }
                 Ok(TypeKind::CustomType(struct_name.clone()))
             }
-            ExprKind::Borrow(var_ref) => {
+            ExprKind::Borrow(var_ref, mutable) => {
                 let existing = state
                     .or_else(
                         state
@@ -651,7 +651,7 @@ impl Expression {
                     var_ref.2,
                 );
 
-                Ok(TypeKind::Borrow(Box::new(var_ref.0.clone())))
+                Ok(TypeKind::Borrow(Box::new(var_ref.0.clone()), *mutable))
             }
             ExprKind::Deref(var_ref) => {
                 let existing = state
@@ -669,7 +669,7 @@ impl Expression {
                     .resolve_ref(typerefs);
 
                 // Update typing to be more accurate
-                let TypeKind::Borrow(inner) = state.or_else(
+                let TypeKind::Borrow(inner, mutable) = state.or_else(
                     var_ref.0.resolve_ref(typerefs).collapse_into(&existing),
                     TypeKind::Vague(Vague::Unknown),
                     var_ref.2,
@@ -677,7 +677,7 @@ impl Expression {
                     return Err(ErrorKind::AttemptedDerefNonBorrow(var_ref.1.clone()));
                 };
 
-                var_ref.0 = TypeKind::Borrow(inner.clone());
+                var_ref.0 = TypeKind::Borrow(inner.clone(), mutable);
 
                 Ok(*inner)
             }
@@ -761,9 +761,10 @@ impl Collapsable for TypeKind {
             (TypeKind::Vague(Vague::Unknown), other) | (other, TypeKind::Vague(Vague::Unknown)) => {
                 Ok(other.clone())
             }
-            (TypeKind::Borrow(val1), TypeKind::Borrow(val2)) => {
-                Ok(TypeKind::Borrow(Box::new(val1.collapse_into(val2)?)))
-            }
+            (TypeKind::Borrow(val1, mut1), TypeKind::Borrow(val2, mut2)) => Ok(TypeKind::Borrow(
+                Box::new(val1.collapse_into(val2)?),
+                *mut1 && *mut2,
+            )),
             _ => Err(ErrorKind::TypesIncompatible(self.clone(), other.clone())),
         }
     }
diff --git a/reid/src/mir/typeinference.rs b/reid/src/mir/typeinference.rs
index 5e02dc0..f00c38a 100644
--- a/reid/src/mir/typeinference.rs
+++ b/reid/src/mir/typeinference.rs
@@ -364,7 +364,7 @@ impl Expression {
                     .from_type(&TypeKind::CustomType(struct_name.clone()))
                     .unwrap())
             }
-            ExprKind::Borrow(var) => {
+            ExprKind::Borrow(var, mutable) => {
                 // Find variable type
                 let type_ref = type_refs
                     .find_var(&var.1)
@@ -377,7 +377,7 @@ impl Expression {
                 }
 
                 Ok(type_refs
-                    .from_type(&TypeKind::Borrow(Box::new(var.0.clone())))
+                    .from_type(&TypeKind::Borrow(Box::new(var.0.clone()), *mutable))
                     .unwrap())
             }
             ExprKind::Deref(var) => {
@@ -393,7 +393,7 @@ impl Expression {
                 }
 
                 match &var.0.resolve_weak(type_refs.types) {
-                    Borrow(type_kind) => Ok(type_refs.from_type(&type_kind).unwrap()),
+                    Borrow(type_kind, _) => Ok(type_refs.from_type(&type_kind).unwrap()),
                     _ => Err(ErrorKind::AttemptedDerefNonBorrow(var.1.clone())),
                 }
             }
diff --git a/reid/src/mir/typerefs.rs b/reid/src/mir/typerefs.rs
index 15230cb..885c953 100644
--- a/reid/src/mir/typerefs.rs
+++ b/reid/src/mir/typerefs.rs
@@ -163,10 +163,10 @@ impl<'outer> ScopeTypeRefs<'outer> {
                 self.types
                     .new(&TypeKind::Array(Box::new(elem_ty.as_type()), *length))
             }
-            TypeKind::Borrow(ty) => {
+            TypeKind::Borrow(ty, mutable) => {
                 let inner_ty = self.from_type(ty)?;
                 self.types
-                    .new(&&TypeKind::Borrow(Box::new(inner_ty.as_type())))
+                    .new(&&TypeKind::Borrow(Box::new(inner_ty.as_type()), *mutable))
             }
             _ => {
                 if let Some(ty_ref) = self.types.find(ty) {
diff --git a/reid_src/borrow.reid b/reid_src/borrow.reid
index ac52245..861caa8 100644
--- a/reid_src/borrow.reid
+++ b/reid_src/borrow.reid
@@ -4,7 +4,7 @@
 fn main() -> u32 {
   let mut value = [4, 3, 2];
 
-  let mut borrow = &value;
+  let mut borrow = &mut value;
   (*borrow)[1] = 17;
 
   return value[1];
diff --git a/reid_src/borrow_hard.reid b/reid_src/borrow_hard.reid
index 87545b5..38fe32e 100644
--- a/reid_src/borrow_hard.reid
+++ b/reid_src/borrow_hard.reid
@@ -1,13 +1,13 @@
 // Arithmetic, function calls and imports!
 
-fn changer(param: &u32) {
+fn changer(param: &mut u32) {
   *param = 17;
 }
 
 fn main() -> u32 {
   let mut value = 6;
 
-  changer(&value);
+  changer(&mut value);
 
   return value;
 }