diff --git a/src/lexer.rs b/src/lexer.rs
index 8c07890..e246b40 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -15,7 +15,7 @@ pub enum Token {
     #[token("else")]      Else,
     #[token("in")]        In,
     #[token("let")]       Let,
-    #[token("var")]       Var,
+    #[token("mut")]       Mut,
     #[token("interface")] Interface,
     #[token("impl")]      Impl,
 
diff --git a/src/lib.rs b/src/lib.rs
index d88a362..49b111f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -30,7 +30,7 @@ impl ColorTheme {
     pub fn token_style(&self, token: &Token) -> &Style {
         use Token::*;
         match token {
-            Struct | Function | For | If | Else | In | Let | Var => &self.keyword,
+            Struct | Function | For | If | Else | In | Let | Mut => &self.keyword,
             BinaryInteger | OctalInteger | DecimalInteger => &self.literal,
             SingleLineComment => &self.comment,
             Error => &self.error,
diff --git a/src/parse.rs b/src/parse.rs
index 7275110..61f62b4 100644
--- a/src/parse.rs
+++ b/src/parse.rs
@@ -13,12 +13,20 @@ impl<'a> ParseTree<'a> {
         let mut declarations = Vec::new();
         let mut associated_struct = None;
         while let Some(tok) = lexer.next() {
-            if let Some(_) = associated_struct {
-                if tok != Token::Function {
-                    lexer.panic_message("Expected fn");
+            if let Some(struct_name) = associated_struct {
+                let mut mutable = false;
+                if tok == Token::Mut {
+                    mutable = true;
+                    lexer.eat_expect(Token::Function);
+                } else if tok != Token::Function {
+                    lexer.panic_message("Expected fn or mut");
                 }
 
-                declarations.push(Definition::build_function(associated_struct, lexer));
+                declarations.push(Definition::build_function(
+                    Some((struct_name, mutable)),
+                    lexer,
+                ));
+
                 associated_struct = None;
             } else {
                 match tok {
@@ -26,7 +34,7 @@ impl<'a> ParseTree<'a> {
                     Token::Function => declarations.push(Definition::build_function(None, lexer)),
                     Token::Interface => declarations.push(Definition::build_interface(lexer)),
                     Token::Identifier => associated_struct = Some(lexer.slice()),
-                    _ => lexer.panic_message("Expected associated struct identifier, fn or struct"),
+                    _ => lexer.panic_message("Expected function, struct, or interface"),
                 }
             }
         }
@@ -42,7 +50,7 @@ pub enum Definition<'a> {
         members: Vec<StructMember<'a>>,
     },
     Function {
-        associated_struct: Option<&'a str>,
+        associated_struct: Option<(&'a str, bool)>,
         name: &'a str,
         signature: FnSig<'a>,
         body: FnBody<'a>,
@@ -78,7 +86,10 @@ impl<'a> Definition<'a> {
         Self::Struct { name, members }
     }
 
-    pub fn build_function(associated_struct: Option<&'a str>, lexer: &mut Lexer<'a>) -> Self {
+    pub fn build_function(
+        associated_struct: Option<(&'a str, bool)>,
+        lexer: &mut Lexer<'a>,
+    ) -> Self {
         let name = lexer.eat_expect_id();
         let (signature, tok) = FnSig::build(lexer);
 
@@ -204,15 +215,6 @@ impl<'a> FnBody<'a> {
                         _ => lexer.panic_message("Unexpected token"),
                     }
                 }
-                Token::Var => {
-                    let (statement, next) = Statement::build_var(lexer);
-                    statements.push(statement);
-                    match next {
-                        Token::Semicolon => {}
-                        Token::BraceClose => break,
-                        _ => lexer.panic_message("Unexpected token"),
-                    }
-                }
                 Token::Identifier => match Expr::build_start(tok, lexer) {
                     (None, Token::Semicolon | Token::BraceClose) => {}
                     (Some(expr), Token::Semicolon) => statements.push(Statement::Expr(expr)),
@@ -223,7 +225,7 @@ impl<'a> FnBody<'a> {
                     _ => lexer.panic_message("Unexpected token"),
                 },
                 Token::BraceClose => break,
-                _ => lexer.panic_message("Expected var, let, or assign"),
+                _ => lexer.panic_message("Expected let or expression"),
             }
         }
 
@@ -294,6 +296,7 @@ pub enum BinaryOp {
     Sub,
     Mul,
     Div,
+    Assign,
     Less,
     LessEq,
     Greater,
@@ -311,6 +314,7 @@ impl BinaryOp {
             OpSub => Some(Sub),
             OpMul => Some(Mul),
             OpDiv => Some(Div),
+            OpAssign => Some(Assign),
             OpLess => Some(Less),
             OpLessEq => Some(LessEq),
             OpGreater => Some(Greater),
@@ -335,10 +339,7 @@ pub enum Statement<'a> {
     Expr(Expr<'a>),
     Let {
         var: &'a str,
-        expr: Expr<'a>,
-    },
-    Var {
-        var: &'a str,
+        mutable: bool,
         expr: Expr<'a>,
     },
     If {
@@ -356,15 +357,18 @@ impl<'a> Statement<'a> {
     }
 
     pub fn build_let(lexer: &mut Lexer<'a>) -> (Self, Token) {
-        let var = lexer.eat_expect_id();
-        let (expr, tail) = Self::eat_assign(lexer);
-        (Self::Let { var, expr }, tail)
-    }
+        let mut mutable = false;
+        let var = match lexer.next().unwrap() {
+            Token::Identifier => lexer.slice(),
+            Token::Mut => {
+                mutable = true;
+                lexer.eat_expect_id()
+            }
+            _ => lexer.panic_message("Unexpected token"),
+        };
 
-    pub fn build_var(lexer: &mut Lexer<'a>) -> (Self, Token) {
-        let var = lexer.eat_expect_id();
         let (expr, tail) = Self::eat_assign(lexer);
-        (Self::Var { var, expr }, tail)
+        (Self::Let { var, mutable, expr }, tail)
     }
 }
 
diff --git a/src/test/example.fae b/src/test/example.fae
index a0e3f80..ed41b46 100644
--- a/src/test/example.fae
+++ b/src/test/example.fae
@@ -37,9 +37,9 @@ fn count_neighbors(BitArray array, i32 i, i32 j) i32 {
 }
 
 // associated function for the World struct
-World fn set_next(i32 i, i32 j) {
+World mut fn set_next(i32 i, i32 j) {
     // members on this World struct can be accessed with `.`
-    var numdots = .numdots; // mutable variables are defined with `var`
+    let mut numdots = .numdots; // mutable variables are defined with `let mut`
     let neighbors = count_neighbors(.current, i, j);
 
     // `if` statements are expressions
@@ -61,7 +61,6 @@ World fn set_next(i32 i, i32 j) {
     };
 
     if next != 0 {
-        // TODO: mutability rules for arguments?
         if i < .xmin { .xmin = i; }
         if i > .xmax { .xmax = i; }
         if j < .ymin { .ymin = j; }
@@ -85,7 +84,6 @@ World fn next_cycle() {
     // TODO: figure out better range definitions, or ditch them altogether
     for y in 0..(.ysize) {
         for x in 0..(.xsize) {
-            // TODO: mutability rules
             .current.set_bit(x, y, .next.get_bit(x, y));
         }
     }
diff --git a/src/test/function.fae b/src/test/function.fae
index 5b939e1..673e652 100644
--- a/src/test/function.fae
+++ b/src/test/function.fae
@@ -3,9 +3,9 @@ fn free_floating(i32 arg1, i32 arg2) ReturnType {
 }
 
 // associated function for the World struct
-World fn set_next(i32 i, i32 j) {
+World mut fn set_next(i32 i, i32 j) {
     // members on this World struct can be accessed with `.`
-    var numdots = .numdots; // mutable variables are defined with `var`
+    let mut numdots = .numdots; // mutable variables are defined with `let mut`
     let neighbors = count_neighbors(.current, i, j);
 
     // `if` statements are expressions
@@ -27,7 +27,6 @@ World fn set_next(i32 i, i32 j) {
     };
 
     if next != 0 {
-        // TODO: mutability rules for arguments?
         if i < .xmin { .xmin = i; }
         if i > .xmax { .xmax = i; }
         if j < .ymin { .ymin = j; }