diff --git a/Cargo.lock b/Cargo.lock
index 462e306..a26e5bb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,12 +2,42 @@
 # It is not intended for manual editing.
 version = 3
 
+[[package]]
+name = "addr2line"
+version = "0.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b9ecd88a8c8378ca913a680cd98f0f13ac67383d35993f86c90a70e3f137816b"
+dependencies = [
+ "gimli",
+]
+
+[[package]]
+name = "adler"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f26201604c87b1e01bd3d98f8d5d9a8fcbb815e8cedb41ffccbeb4bf593a35fe"
+
 [[package]]
 name = "autocfg"
 version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa"
 
+[[package]]
+name = "backtrace"
+version = "0.3.66"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cab84319d616cfb654d03394f38ab7e6f0919e181b1b57e1fd15e7fb4077d9a7"
+dependencies = [
+ "addr2line",
+ "cc",
+ "cfg-if",
+ "libc",
+ "miniz_oxide",
+ "object",
+ "rustc-demangle",
+]
+
 [[package]]
 name = "bitflags"
 version = "1.3.2"
@@ -41,6 +71,7 @@ dependencies = [
 name = "complexpr"
 version = "0.1.0"
 dependencies = [
+ "backtrace",
  "lazy_static",
  "num-complex",
  "num-rational",
@@ -128,6 +159,12 @@ dependencies = [
  "wasi",
 ]
 
+[[package]]
+name = "gimli"
+version = "0.26.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "22030e2c5a68ec659fde1e949a745124b48e6fa8b045b7ed5bd1fe4ccc5c4e5d"
+
 [[package]]
 name = "io-lifetimes"
 version = "0.7.3"
@@ -167,6 +204,15 @@ version = "2.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2dffe52ecf27772e601905b7522cb4ef790d2cc203488bbd0e2fe85fcb74566d"
 
+[[package]]
+name = "miniz_oxide"
+version = "0.5.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96590ba8f175222643a85693f33d26e9c8a015f599c216509b1a6894af675d34"
+dependencies = [
+ "adler",
+]
+
 [[package]]
 name = "nibble_vec"
 version = "0.1.0"
@@ -238,6 +284,15 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "object"
+version = "0.29.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "21158b2c33aa6d4561f1c0a6ea283ca92bc54802a93b263e910746d679a7eb53"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "proc-macro2"
 version = "1.0.43"
@@ -286,6 +341,12 @@ dependencies = [
  "thiserror",
 ]
 
+[[package]]
+name = "rustc-demangle"
+version = "0.1.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7ef03e0a2b150c7a90d01faf6254c9c48a41e95fb2a8c2ac1c6f0d2b9aefc342"
+
 [[package]]
 name = "rustix"
 version = "0.35.9"
diff --git a/Cargo.toml b/Cargo.toml
index a7dd19d..c3b8954 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,3 +9,4 @@ num-complex = "0.4.1"
 num-rational = "0.4.0"
 num-traits = "0.2.15"
 rustyline = "10.0.0"
+backtrace = "0.3.66"
diff --git a/examples/cat.cxpr b/examples/cat.cxpr
index d46429d..ffaf3c9 100644
--- a/examples/cat.cxpr
+++ b/examples/cat.cxpr
@@ -1,3 +1,3 @@
 while true { 
     print(input());
-}
\ No newline at end of file
+}
diff --git a/examples/collatz.cxpr b/examples/collatz.cxpr
index f2fb77a..ae0d830 100644
--- a/examples/collatz.cxpr
+++ b/examples/collatz.cxpr
@@ -7,4 +7,4 @@ while n != 1 {
         n = 3*n+1;
     }
 }
-println(n);
\ No newline at end of file
+println(n);
diff --git a/examples/factorial.cxpr b/examples/factorial.cxpr
new file mode 100644
index 0000000..cfc5a5e
--- /dev/null
+++ b/examples/factorial.cxpr
@@ -0,0 +1,11 @@
+fn factorial(n) {
+    if n <= 1 {
+        return 1;
+    } else {
+        return n * factorial(n-1);
+    }
+}
+
+for n : range(0, 10) {
+    println(factorial(n));
+}
diff --git a/examples/fib.cxpr b/examples/fib.cxpr
index 4e39cd3..4f34716 100644
--- a/examples/fib.cxpr
+++ b/examples/fib.cxpr
@@ -6,4 +6,4 @@ while z < 1000000 {
     z = y;
     y = x;
     println(z);
-}
\ No newline at end of file
+}
diff --git a/examples/helloworld.cxpr b/examples/helloworld.cxpr
index ce50212..0b449fa 100644
--- a/examples/helloworld.cxpr
+++ b/examples/helloworld.cxpr
@@ -1,2 +1,2 @@
 # Prints the string "Hello, world!" followed by a newline
-println("Hello, world!");
\ No newline at end of file
+println("Hello, world!");
diff --git a/examples/mbrot.cxpr b/examples/mbrot.cxpr
index ec6a8c8..f958182 100644
--- a/examples/mbrot.cxpr
+++ b/examples/mbrot.cxpr
@@ -36,4 +36,4 @@ for y: range(0,yscale) {
         }
     }
     println("");
-}
\ No newline at end of file
+}
diff --git a/src/bin/main.rs b/src/bin/main.rs
index 117378e..539f23d 100644
--- a/src/bin/main.rs
+++ b/src/bin/main.rs
@@ -1,5 +1,6 @@
-use std::{rc::Rc, cell::RefCell, fs};
+use std::{rc::Rc, cell::RefCell, fs, panic::{self, PanicInfo}, thread::Thread};
 
+use backtrace::Backtrace;
 use complexpr::{eval::Environment, interpreter::interpret, value::Value, stdlib};
 use rustyline::{self, error::ReadlineError};
 
@@ -7,7 +8,24 @@ const C_RESET: &str = "\x1b[0m";
 const C_BLUE: &str = "\x1b[94m";
 const PROMPT: &str = "\x1b[94m>> \x1b[0m";
 
+fn panic_hook(info: &PanicInfo) {
+    eprintln!("{:?}", Backtrace::new());
+    eprintln!("!!! Internal interpreter error occured !!!");
+    if let Some(s) = std::thread::current().name() {
+        eprintln!("Thread: {}", s);
+    }
+    if let Some(loc) = info.location() {
+        eprintln!("Location: {}:{}:{}", loc.file(), loc.line(), loc.column())
+    }
+    if let Some(s) = info.payload().downcast_ref::<&str>() {
+        eprintln!("Message: {}", s);
+    } else if let Some(s) = info.payload().downcast_ref::<String>() {
+        eprintln!("Message: {}", s);
+    }
+}
+
 fn main() -> Result<(), Box<dyn std::error::Error>> {
+    panic::set_hook(Box::new(panic_hook));
     let args: Vec<String> = std::env::args().collect();
     if args.len() == 2 {
         let fname = &args[1];
@@ -35,7 +53,7 @@ fn repl() -> Result<(), Box<dyn std::error::Error>> {
                 match result {
                     Ok(Value::Nil) => (),
                     Ok(value) => println!("{}", value.repr()),
-                    Err(e) => println!("{}", e)
+                    Err(e) => print!("{}", e)
                 }
             }
             Err(ReadlineError::Eof) => break,
diff --git a/src/eval.rs b/src/eval.rs
index 30b4d4a..b3c26b2 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -1,6 +1,6 @@
 use std::{collections::HashMap, rc::Rc, cell::RefCell};
 
-use crate::{value::{Value, Complex}, expr::{Stmt, Expr}, token::{TokenType, Token, OpType}, RuntimeError, Position};
+use crate::{value::{Value, Complex, Func}, expr::{Stmt, Expr}, token::{TokenType, Token, OpType}, RuntimeError, Position};
 
 #[derive(Debug)]
 pub struct Environment {
@@ -54,17 +54,21 @@ impl Default for Environment {
     }
 }
 
+#[derive(Debug)]
 pub enum Unwind {
-    Continue{pos: Position}, Break{pos: Position}, Return{pos: Position, value: Value}, Error(RuntimeError)
+    Continue{pos: Position}, 
+    Break{pos: Position}, 
+    Return{pos: Position, value: Value}, 
+    Error(RuntimeError)
 }
 
 impl Unwind {
     pub fn as_error(self) -> RuntimeError {
         match self {
             Self::Error(e) => e,
-            Self::Continue { pos } => RuntimeError { message: "continue statement outside of loop".into(), pos },
-            Self::Break { pos } => RuntimeError { message: "break statement outside of loop".into(), pos },
-            Self::Return { pos, .. } => RuntimeError { message: "return statement outside of function".into(), pos },
+            Self::Continue { pos } => RuntimeError::new("continue statement outside of loop", pos),
+            Self::Break { pos } => RuntimeError::new("break statement outside of loop", pos),
+            Self::Return { pos, .. } => RuntimeError::new("return statement outside of function", pos),
         }
     }
 }
@@ -86,7 +90,7 @@ fn unwrap_ident_token(tok: &Token) -> &Rc<str> {
 pub fn eval_stmt(stmt: &Stmt, env: EnvRef) -> Result<(), Unwind> {
     match stmt {
         Stmt::Expr{ expr } 
-            => drop(eval_expr(expr, env)),
+            => drop(eval_expr(expr, env)?),
         Stmt::Let { lhs, rhs: None } 
             => env.borrow_mut().declare(unwrap_ident_token(lhs).clone(), Value::Nil),
         Stmt::Let { lhs, rhs: Some(rhs) } => {
@@ -95,8 +99,8 @@ pub fn eval_stmt(stmt: &Stmt, env: EnvRef) -> Result<(), Unwind> {
         },
         Stmt::Block { stmts } => {
             let block_env = Environment::extend(env).wrap();
-            for stmt in stmts { 
-                eval_stmt(stmt, block_env.clone())? 
+            for stmt in stmts {
+                eval_stmt(stmt, block_env.clone())?;
             }
         },
         Stmt::If { if_clauses, else_clause } => {
@@ -116,7 +120,7 @@ pub fn eval_stmt(stmt: &Stmt, env: EnvRef) -> Result<(), Unwind> {
             env.borrow_mut().declare(name.clone(), Value::Nil);
             let iterator = iter.iter();
             if let Err(e) = iterator {
-                return Err(RuntimeError { message: e, pos: var.pos.clone() }.into())
+                return Err(RuntimeError::new(e, var.pos.clone()).into())
             }
             if let Ok(i) = iterator {
                 for v in i {
@@ -145,8 +149,22 @@ pub fn eval_stmt(stmt: &Stmt, env: EnvRef) -> Result<(), Unwind> {
                 }
             }
         },
+        Stmt::Fn { name, args, body } => {
+            let name = name.ty.clone().as_ident().unwrap();
+            let func = Func { 
+                name: Some(name.clone()),
+                args: args.into_iter().map(|a| a.ty.clone().as_ident().unwrap()).collect(),
+                env: env.clone(),
+                func: body.as_ref().clone()
+            };
+            env.borrow_mut().declare(name, Value::Func(func));
+        },
         Stmt::Break { tok } => return Err(Unwind::Break { pos: tok.pos.clone() }),
         Stmt::Continue { tok } => return Err(Unwind::Continue { pos: tok.pos.clone() }),
+        Stmt::Return { tok, expr } => {
+            let value = eval_expr(expr, env)?;
+            return Err(Unwind::Return { pos: tok.pos.clone(), value })
+        }
     }
     Ok(())
 }
@@ -186,7 +204,16 @@ pub fn eval_expr(expr: &Expr, env: EnvRef) -> Result<Value, RuntimeError> {
         Expr::Index { lhs, index, pos } => {
             let l = eval_expr(lhs, env.clone())?;
             let idx = eval_expr(index, env)?;
-            l.index(&idx).map_err(|e| RuntimeError { message: e, pos: pos.clone() })
+            l.index(&idx).map_err(|e| RuntimeError::new(e, pos.clone()))
+        },
+        Expr::Fn { args, body } => {
+            let func = Func { 
+                name: None,
+                args: args.into_iter().map(|a| a.ty.clone().as_ident().unwrap()).collect(),
+                env: env.clone(),
+                func: body.as_ref().clone()
+            };
+            Ok(Value::Func(func))
         },
     }
 }
@@ -209,7 +236,7 @@ pub fn eval_ident(token: &Token, env: EnvRef) -> Result<Value, RuntimeError> {
     if let Token { ty: TokenType::Ident(name), ..} = token {
         env.borrow_mut()
             .get(name)
-            .ok_or_else(|| RuntimeError { message: "Variable not defined in scope".into(), pos: token.pos.clone() })
+            .ok_or_else(|| RuntimeError::new("Variable not defined in scope", token.pos.clone()))
     } else { 
         unreachable!() 
     }
@@ -223,13 +250,13 @@ pub fn eval_assignment(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Resul
             let r = eval_expr(rhs, env.clone())?;
             env.borrow_mut()
                 .set(name.clone(), r)
-                .map_err(|_| RuntimeError { message: "Variable not declared before assignment".into(), pos: op.pos.clone() })?;
+                .map_err(|_| RuntimeError::new("Variable not declared before assignment", op.pos.clone()))?;
             Ok(Value::Nil)
         } else {
             // compound assignment
             let prev_value = env.borrow_mut()
                 .get(name)
-                .ok_or_else(|| RuntimeError { message: "Variable not defined in scope".into(), pos: op.pos.clone()})?;
+                .ok_or_else(|| RuntimeError::new("Variable not defined in scope", op.pos.clone()))?;
             let r = eval_expr(rhs, env.clone())?;
 
             let result = match op.ty {
@@ -239,7 +266,7 @@ pub fn eval_assignment(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Resul
                 TokenType::SlashEqual => &prev_value / &r,
                 TokenType::PercentEqual => &prev_value % &r,
                 _ => todo!() // TODO more operations
-            }.map_err(|e| RuntimeError { message: e, pos: op.pos.clone() })?;
+            }.map_err(|e| RuntimeError::new(e, op.pos.clone()))?;
 
             env.borrow_mut()
                 .set(name.clone(), result.clone()).expect("unreachable");
@@ -250,10 +277,10 @@ pub fn eval_assignment(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Resul
         let idx = eval_expr(index, env.clone())?;
         if op.ty == TokenType::Equal {
             let r = eval_expr(rhs, env)?;
-            l.assign_index(&idx, r.clone()).map_err(|e| RuntimeError { message: e, pos: pos.clone() })?;
+            l.assign_index(&idx, r.clone()).map_err(|e| RuntimeError::new(e, pos.clone()))?;
             Ok(r)
         } else {
-            let prev_value = l.index(&idx).map_err(|e| RuntimeError { message: e, pos: pos.clone() })?;
+            let prev_value = l.index(&idx).map_err(|e| RuntimeError::new(e, pos.clone()))?;
             let r = eval_expr(rhs, env)?;
             let result = match op.ty {
                 TokenType::PlusEqual => &prev_value + &r,
@@ -262,8 +289,8 @@ pub fn eval_assignment(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Resul
                 TokenType::SlashEqual => &prev_value / &r,
                 TokenType::PercentEqual => &prev_value % &r,
                 _ => todo!() // TODO more operations
-            }.map_err(|e| RuntimeError { message: e, pos: op.pos.clone() })?;
-            l.assign_index(&idx, result.clone()).map_err(|e| RuntimeError { message: e, pos: pos.clone() })?;
+            }.map_err(|e| RuntimeError::new(e, op.pos.clone()))?;
+            l.assign_index(&idx, result.clone()).map_err(|e| RuntimeError::new(e, pos.clone()))?;
             Ok(result)
         }
     } else {
@@ -281,7 +308,7 @@ pub fn eval_binary(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Result<Va
         TokenType::Slash => &l / &r,
         TokenType::Percent => &l % &r,
         _ => todo!() // TODO other operations
-    }.map_err(|e| RuntimeError { message: e, pos: op.pos.clone() })
+    }.map_err(|e| RuntimeError::new(e, op.pos.clone()))
 }
 
 pub fn eval_boolean(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Result<Value, RuntimeError> {
@@ -304,10 +331,10 @@ pub fn eval_boolean(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Result<V
 pub fn eval_comp(lhs: &Expr, rhs: &Expr, op: &Token, env: EnvRef) -> Result<Value, RuntimeError> {
     let l = eval_expr(lhs, env.clone())?;
     let r = eval_expr(rhs, env)?;
-    let mk_err = || RuntimeError {
-        message: format!("Cannot compare {:?} with {:?}", l, r),
-        pos: op.pos.clone()
-    };
+    let mk_err = || RuntimeError::new(
+        format!("Cannot compare {:?} with {:?}", l, r),
+        op.pos.clone()
+    );
     match op.ty {
         TokenType::DoubleEqual => Ok(Value::Bool(l == r)),
         TokenType::BangEqual => Ok(Value::Bool(l != r)),
@@ -336,5 +363,5 @@ pub fn eval_unary(arg: &Expr, op: &Token, env: EnvRef) -> Result<Value, RuntimeE
         TokenType::Minus => -a,
         TokenType::Bang => Ok(Value::Bool(!a.truthy())),
         _ => todo!(),
-    }.map_err(|e| RuntimeError { message: e, pos: op.pos.clone() })
+    }.map_err(|e| RuntimeError::new(e, op.pos.clone()))
 }
diff --git a/src/expr.rs b/src/expr.rs
index 0d98b5c..8d35107 100644
--- a/src/expr.rs
+++ b/src/expr.rs
@@ -2,6 +2,7 @@ use std::fmt;
 
 use crate::{token::{Token, OpType}, Position};
 
+#[derive(Clone)]
 pub enum Stmt {
     Expr { expr: Expr },
     Let { lhs: Token, rhs: Option<Expr> },
@@ -11,6 +12,8 @@ pub enum Stmt {
     While { expr: Expr, stmt: Box<Stmt> },
     Break { tok: Token },
     Continue { tok: Token },
+    Return { tok: Token, expr: Expr },
+    Fn { name: Token, args: Vec<Token>, body: Box<Stmt> },
 }
 
 impl fmt::Debug for Stmt {
@@ -18,11 +21,13 @@ impl fmt::Debug for Stmt {
         match self {
             Self::Expr { expr } => write!(f, "{:?}", expr),
             Self::Let { lhs, rhs } => write!(f, "(let {:?} = {:?})", lhs, rhs),
+            Self::Block { stmts } => write!(f, "(block {:?})", stmts),
             _ => todo!()
         }
     }
 }
 
+#[derive(Clone)]
 pub enum Expr {
     Binary { lhs: Box<Expr>, rhs: Box<Expr>, op: Token },
     Unary { arg: Box<Expr>, op: Token },
@@ -31,6 +36,7 @@ pub enum Expr {
     List { items: Vec<Expr> },
     FuncCall { func: Box<Expr>, args: Vec<Expr>, pos: Position },
     Index { lhs: Box<Expr>, index: Box<Expr>, pos: Position },
+    Fn { args: Vec<Token>, body: Box<Stmt> },
 }
 
 impl fmt::Debug for Expr {
@@ -43,6 +49,7 @@ impl fmt::Debug for Expr {
             Self::List { items } => write!(f, "(list {:?})", items),
             Self::FuncCall { func, args, .. } => write!(f, "(call {:?} {:?})", func, args),
             Self::Index { lhs, index, .. } => write!(f, "(index {:?} {:?})", lhs, index),
+            Self::Fn { args, body } => write!(f, "(fn {:?} {:?})", args, body)
         }
     }
 }
diff --git a/src/interpreter.rs b/src/interpreter.rs
index 928ab0c..36cfefc 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -3,7 +3,8 @@ use std::{cell::RefCell, rc::Rc};
 use crate::{value::Value, lexer::Lexer, parser::Parser, eval::{Environment, eval_stmt, eval_expr, EnvRef}, expr::Stmt, stdlib};
 
 pub fn interpret(src: &str, fname: Option<String>, env: Option<EnvRef>, repl: bool) -> Result<Value, Box<dyn std::error::Error>> {
-    let mut lexer = Lexer::new(src, fname);
+    let ctx_name = if repl { "<interactive input>" } else { fname.as_ref().map(|s| s.as_ref()).unwrap_or("<unknown>") };
+    let mut lexer = Lexer::new(src, fname.clone());
     lexer.lex()?;
     let mut parser = Parser::new(lexer.into_tokens(), repl);
     let ast = parser.parse()?;
@@ -18,9 +19,9 @@ pub fn interpret(src: &str, fname: Option<String>, env: Option<EnvRef>, repl: bo
     let mut result = Value::Nil;
     for stmt in ast {
         if let Stmt::Expr{expr} = stmt {
-            result = eval_expr(&expr, environ.clone())?;
+            result = eval_expr(&expr, environ.clone()).map_err(|e| e.finish(Some(Rc::from(ctx_name))))?;
         } else {
-            eval_stmt(&stmt, environ.clone()).map_err(|e| e.as_error())?;
+            eval_stmt(&stmt, environ.clone()).map_err(|e| e.as_error().finish(Some(Rc::from(ctx_name))))?;
             result = Value::Nil;
         }
     }
diff --git a/src/lexer.rs b/src/lexer.rs
index 02fcb83..8c57632 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -268,6 +268,7 @@ impl Lexer {
             "else" => TokenType::Else,
             "while" => TokenType::While,
             "for" => TokenType::For,
+            "fn" => TokenType::Fn,
             "let" => TokenType::Let,
             "break" => TokenType::Break,
             "continue" => TokenType::Continue,
diff --git a/src/lib.rs b/src/lib.rs
index f38afa2..8bf4e1f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -24,10 +24,40 @@ pub struct ParserError {
     pub pos: Position
 }
 
+#[derive(Debug)]
+pub struct Stackframe {
+    pub pos: Position,
+    pub fn_name: Option<Rc<str>>,
+}
+
 #[derive(Debug)]
 pub struct RuntimeError {
     pub message: String,
-    pub pos: Position
+    pub stacktrace: Vec<Stackframe>,
+    pub last_pos: Option<Position>,
+}
+
+impl RuntimeError {
+    pub fn new<S>(message: S, pos: Position) -> Self
+    where S: Into<String> {
+        Self { 
+            message: message.into(), 
+            stacktrace: vec![],
+            last_pos: Some(pos)
+        }
+    }
+
+    pub fn exit_fn(mut self, fn_name: Option<Rc<str>>, pos: Position) -> Self {
+        self.stacktrace.push(Stackframe { pos: self.last_pos.unwrap(), fn_name });
+        self.last_pos = Some(pos);
+        self
+    }
+
+    pub fn finish(mut self, ctx_name: Option<Rc<str>>) -> Self {
+        self.stacktrace.push(Stackframe { pos: self.last_pos.unwrap(), fn_name: ctx_name });
+        self.last_pos = None;
+        self
+    }
 }
 
 impl fmt::Display for ParserError {
@@ -43,12 +73,16 @@ impl fmt::Display for ParserError {
 
 impl fmt::Display for RuntimeError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "Error: {}\n    In {} at {},{}", 
-            self.message, 
-            self.pos.file.as_ref().map(|o| o.as_ref()).unwrap_or("<unknown>"), 
-            self.pos.line, 
-            self.pos.col
-        )
+        writeln!(f, "Error: {}", self.message)?;
+        for frame in &self.stacktrace {
+            writeln!(f, "    In {} at {}:{}:{}", 
+                frame.fn_name.as_ref().map(|o| o.as_ref()).unwrap_or("<anonymous fn>"),
+                frame.pos.file.as_ref().map(|o| o.as_ref()).unwrap_or("<unknown>"),
+                frame.pos.line,
+                frame.pos.col
+            )?;
+        }
+        Ok(())
     }
 }
 
diff --git a/src/parser.rs b/src/parser.rs
index 13c530c..20d138c 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -82,11 +82,20 @@ impl Parser {
             TokenType::Break => {
                 let tok = self.next();
                 self.terminate_stmt(Stmt::Break{ tok })
-            }
+            },
             TokenType::Continue => {
                 let tok = self.next();
                 self.terminate_stmt(Stmt::Continue{ tok })
-            }
+            },
+            TokenType::Return => {
+                let tok = self.next();
+                let expr = self.assignment()?;
+                self.terminate_stmt(Stmt::Return{ tok, expr })
+            },
+            TokenType::Fn => {
+                let tok = self.next();
+                self.fndef()
+            },
             _ => {
                 // fallback to an expression terminated with a semicolon
                 let expr = self.assignment()?;
@@ -201,6 +210,36 @@ impl Parser {
         Ok(Stmt::While{ expr, stmt: Box::new(stmt) })
     }
 
+    fn fndef(&mut self) -> Result<Stmt, ParserError> {
+        self.err_on_eof()?;
+        let name = self.next();
+        let name = if let TokenType::Ident(_) = name.ty {
+            name
+        } else {
+            return Err(ParserError { message: "Expected identifer in function declaration".into(), pos: name.pos })
+        };
+        self.err_on_eof()?;
+        let next = self.next();
+        if let TokenType::LParen = next.ty {} else {
+            return Err(ParserError { message: "Expected left parenthesis to start arguments list".into(), pos: next.pos })
+        }
+        let args = self.commalist(TokenType::RParen, Self::ident)?.into_iter().map(|e| {
+            if let Expr::Ident { value: i } = e { i }
+            else { panic!() }
+        }).collect();
+        self.err_on_eof()?;
+        let body = self.statement()?;
+        Ok(Stmt::Fn { name, args, body: Box::new(body) })
+    }
+
+    fn ident(&mut self) -> Result<Expr, ParserError> {
+        let next = self.next();
+        match next.ty {
+            TokenType::Ident(_) => Ok(Expr::Ident{ value: next }),
+            _ => Err(ParserError { message: "Expected identifier".into(), pos: next.pos })
+        }
+    }
+
     fn block(&mut self) -> Result<Stmt, ParserError> {
         let mut stmts = vec![];
         while !self.at_end() && self.peek().ty != TokenType::RBrace {
@@ -352,6 +391,19 @@ impl Parser {
         } else if next.ty == TokenType::LBrack {
             let items = self.commalist(TokenType::RBrack, Self::assignment)?;
             Ok(Expr::List { items })
+        } else if next.ty == TokenType::Fn {
+            self.err_on_eof()?;
+            let next = self.next();
+            if let TokenType::LParen = next.ty {} else {
+                return Err(ParserError { message: "Expected left parenthesis to start arguments list".into(), pos: next.pos })
+            }
+            let args = self.commalist(TokenType::RParen, Self::ident)?.into_iter().map(|e| {
+                if let Expr::Ident { value: i } = e { i }
+                else { panic!() }
+            }).collect();
+            self.err_on_eof()?;
+            let body = self.statement()?;
+            Ok(Expr::Fn { args, body: Box::new(body) })
         } else {
             Err(self.mk_error(format!("Unexpected token: {:?}", next.ty)))
         }
diff --git a/src/stdlib/mod.rs b/src/stdlib/mod.rs
index ef103d8..5127f84 100644
--- a/src/stdlib/mod.rs
+++ b/src/stdlib/mod.rs
@@ -2,32 +2,32 @@ use std::{rc::Rc, io::Write, cmp::Ordering};
 
 use num_traits::ToPrimitive;
 
-use crate::{value::{Value, BuiltinFn}, eval::Environment};
+use crate::{value::{Value, BuiltinFunc}, eval::Environment};
 
 pub fn load(env: &mut Environment) {
     let mut name: Rc<str>;
     name = Rc::from("str");
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_str, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_str, arg_count: 1, name }));
     name = Rc::from("repr");
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_repr, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_repr, arg_count: 1, name }));
     name = Rc::from("print");
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_print, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_print, arg_count: 1, name }));
     name = Rc::from("println"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_println, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_println, arg_count: 1, name }));
     name = Rc::from("input"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_input, arg_count: 0, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_input, arg_count: 0, name }));
     name = Rc::from("ord"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_ord, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_ord, arg_count: 1, name }));
     name = Rc::from("chr"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_chr, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_chr, arg_count: 1, name }));
     name = Rc::from("range"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_range, arg_count: 2, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_range, arg_count: 2, name }));
     name = Rc::from("len"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_len, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_len, arg_count: 1, name }));
     name = Rc::from("re"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_re, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_re, arg_count: 1, name }));
     name = Rc::from("im"); 
-    env.declare(name.clone(), Value::BuiltinFn(BuiltinFn { func: fn_im, arg_count: 1, name }));
+    env.declare(name.clone(), Value::BuiltinFunc(BuiltinFunc { func: fn_im, arg_count: 1, name }));
 }
 
 fn fn_str(args: Vec<Value>) -> Result<Value, String> {
diff --git a/src/token.rs b/src/token.rs
index 1a2430b..771c376 100644
--- a/src/token.rs
+++ b/src/token.rs
@@ -35,7 +35,7 @@ pub enum TokenType {
 
     True, False, Nil, 
     If, Elif, Else, For, While, 
-    Let, Break, Continue, Return
+    Fn, Let, Break, Continue, Return
 }
 
 impl TokenType {
@@ -63,6 +63,13 @@ impl TokenType {
             _ => None
         }
     }
+
+    pub fn as_ident(self) -> Option<Rc<str>> {
+        match self {
+            Self::Ident(s) => Some(s),
+            _ => None
+        }
+    }
 }
 
 #[derive(Clone,Copy,Debug,PartialEq,Eq)]
diff --git a/src/value.rs b/src/value.rs
index ba7a80d..136f921 100644
--- a/src/value.rs
+++ b/src/value.rs
@@ -2,24 +2,32 @@ use std::{rc::Rc, collections::HashMap, ops::*, fmt, cmp::Ordering, cell::RefCel
 
 use num_traits::{Zero, ToPrimitive};
 
-use crate::{RuntimeError, Position};
+use crate::{RuntimeError, Position, eval::{EnvRef, eval_stmt, Environment, Unwind}, expr::Stmt};
 
 pub type Rational = num_rational::Ratio<i64>;
 pub type Complex = num_complex::Complex64;
 
 #[derive(Clone)]
-pub struct BuiltinFn {
+pub struct BuiltinFunc {
     pub name: Rc<str>,
     pub func: fn(Vec<Value>) -> Result<Value, String>,
     pub arg_count: usize
 }
 
-impl fmt::Debug for BuiltinFn {
+impl fmt::Debug for BuiltinFunc {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("BuiltinFn").field("name", &self.name).field("arg_count", &self.arg_count).finish()
     }
 }
 
+#[derive(Clone, Debug)]
+pub struct Func {
+    pub name: Option<Rc<str>>,
+    pub args: Vec<Rc<str>>,
+    pub env: EnvRef,
+    pub func: Stmt
+}
+
 #[derive(Clone, Debug)]
 pub struct Data {
     pub ty: usize,
@@ -42,7 +50,8 @@ pub enum Value {
     Char(char),
     String(Rc<str>), 
     List(Rc<RefCell<Vec<Value>>>), Map(Rc<HashMap<Value,Value>>),
-    BuiltinFn(BuiltinFn),
+    BuiltinFunc(BuiltinFunc),
+    Func(Func),
     Data(Data),
 }
 
@@ -73,21 +82,44 @@ impl Value {
 
     pub fn call(&self, args: Vec<Value>, pos: &Position) -> Result<Value, RuntimeError> {
         match self {
-            Value::BuiltinFn(f) => {
+            Value::BuiltinFunc(f) => {
                 match args.len().cmp(&f.arg_count) {
                     Ordering::Equal => 
-                        (f.func)(args).map_err(|e| RuntimeError { message: e, pos: pos.clone() }),
-                    Ordering::Less => Err(RuntimeError { 
-                        message: format!("Not enough arguments for function: expected {}, got {}", f.arg_count, args.len()), 
-                        pos: pos.clone() 
-                    }),
-                    Ordering::Greater => Err(RuntimeError { 
-                        message: format!("Too many arguments for function: expected {}, got {}", f.arg_count, args.len()), 
-                        pos: pos.clone() 
-                    })
+                        (f.func)(args).map_err(|e| RuntimeError::new(e, pos.clone())),
+                    Ordering::Less => Err(RuntimeError::new(
+                        format!("Not enough arguments for function: expected {}, got {}", f.arg_count, args.len()), 
+                        pos.clone() 
+                    )),
+                    Ordering::Greater => Err(RuntimeError::new(
+                        format!("Too many arguments for function: expected {}, got {}", f.arg_count, args.len()), 
+                        pos.clone() 
+                    ))
                 }
-            }
-            _ => Err(RuntimeError { message: "Cannot call".into(), pos: pos.clone() })
+            },
+            Value::Func(f) => {
+                match args.len().cmp(&f.args.len()) {
+                    Ordering::Equal => {
+                        let mut env = Environment::extend(f.env.clone());
+                        for (k, v) in f.args.iter().zip(args.iter()) {
+                            env.declare(k.clone(), v.clone());
+                        }
+                        match eval_stmt(&f.func, env.wrap()) {
+                            Ok(()) => Ok(Value::Nil),
+                            Err(Unwind::Return{ value, .. }) => Ok(value),
+                            Err(e) => Err(e.as_error().exit_fn(f.name.clone(), pos.clone()))
+                        }
+                    },
+                    Ordering::Less => Err(RuntimeError::new(
+                        format!("Not enough arguments for function: expected {}, got {}", f.args.len(), args.len()), 
+                        pos.clone() 
+                    )),
+                    Ordering::Greater => Err(RuntimeError::new(
+                        format!("Too many arguments for function: expected {}, got {}", f.args.len(), args.len()), 
+                        pos.clone() 
+                    ))
+                }
+            },
+            _ => Err(RuntimeError::new("Cannot call", pos.clone()))
         }
     }
 
@@ -104,7 +136,11 @@ impl Value {
             Self::List(l) => Rc::from(format!("{:?}", l)), // TODO fix
             Self::Map(m) => Rc::from(format!("{:?}", m)), // TODO fix
             Self::Type(_) => todo!(),
-            Self::BuiltinFn(bf) => Rc::from(format!("<builtin fn {} at {:?}>", bf.name, bf.func as *const ())),
+            Self::BuiltinFunc(bf) => Rc::from(format!("<builtin fn {} at {:?}>", bf.name, bf.func as *const ())),
+            Self::Func(f) => match &f.name {
+                Some(name) => Rc::from(format!("<fn {}>", name)),
+                None => Rc::from("<anonymous fn>"),
+            },
             Self::Data(_) => todo!(),
         }
     }
@@ -122,7 +158,11 @@ impl Value {
             Self::List(l) => Rc::from(format!("{:?}", l.borrow())), // TODO fix
             Self::Map(m) => Rc::from(format!("{:?}", m)), // TODO fix
             Self::Type(_) => todo!(),
-            Self::BuiltinFn(bf) => Rc::from(format!("<builtin fn {} at {:?}>", bf.name, bf.func as *const ())),
+            Self::BuiltinFunc(bf) => Rc::from(format!("<builtin fn {} at {:?}>", bf.name, bf.func as *const ())),
+            Self::Func(f) => match &f.name {
+                Some(name) => Rc::from(format!("<fn {}>", name)),
+                None => Rc::from("<anonymous fn>"),
+            },
             Self::Data(_) => todo!(),
         }
     }
@@ -195,7 +235,7 @@ impl PartialEq for Value {
             (Self::String(a), Self::String(b)) => a == b,
             (Self::List(a), Self::List(b)) => a == b,
             (Self::Map(_), Self::Map(_)) => todo!("Can't test maps for equality yet"),
-            (Self::BuiltinFn(a), Self::BuiltinFn(b)) 
+            (Self::BuiltinFunc(a), Self::BuiltinFunc(b)) 
                 => (a.func as *const ()) == (b.func as *const ()) && a.arg_count == b.arg_count,
             (Self::Data(_), Self::Data(_)) => todo!("Can't compare data yet"),
             _ => false