diff --git a/rust/rox/src/callable.rs b/rust/rox/src/callable.rs
new file mode 100644
index 0000000..2d3898f
--- /dev/null
+++ b/rust/rox/src/callable.rs
@@ -0,0 +1,39 @@
+use crate::{interpreter::Interpreter, value::Value};
+use core::fmt::Debug;
+use std::fmt::Display;
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum CallingError {
+    #[error("Failed calling: {0}.")]
+    CallFailed(String),
+    #[error("Expected {0} arguments but got {1}.")]
+    ArgumentMismatch(usize, usize),
+    #[error("Failed calling: {0}.")]
+    CallInterpretationFailed(String),
+}
+
+/// Implementations can be called.
+pub trait Callable {
+    fn name(&self) -> String;
+    fn arity(&self) -> usize;
+    fn call(&self, interpreter: &mut Interpreter, args: Vec<Value>) -> Result<Value, CallingError>;
+}
+
+impl Debug for dyn Callable {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self)
+    }
+}
+
+impl Display for dyn Callable {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.name())
+    }
+}
+
+impl PartialEq for dyn Callable {
+    fn eq(&self, other: &Self) -> bool {
+        self.name() == other.name() && self.arity() == other.arity()
+    }
+}
diff --git a/rust/rox/src/expression.rs b/rust/rox/src/expression.rs
index 0600328..defaba6 100644
--- a/rust/rox/src/expression.rs
+++ b/rust/rox/src/expression.rs
@@ -12,6 +12,11 @@ pub enum Expression {
         operator: Token,
         right: Box<Expression>,
     },
+    Call {
+        callee: Box<Expression>,
+        paren: Token,
+        args: Vec<Expression>,
+    },
     Grouping {
         expression: Box<Expression>,
     },
diff --git a/rust/rox/src/function.rs b/rust/rox/src/function.rs
new file mode 100644
index 0000000..47e92fe
--- /dev/null
+++ b/rust/rox/src/function.rs
@@ -0,0 +1,61 @@
+use crate::{
+    callable::{Callable, CallingError},
+    environment::Environment,
+    interpreter::Interpreter,
+    statement::Statement,
+    token::Token,
+    value::Value,
+};
+use core::fmt::Debug;
+use std::{cell::RefCell, fmt::Display, rc::Rc};
+
+/// A lox function.
+#[derive(Debug, Clone)]
+pub struct Function {
+    pub name: Token,
+    pub params: Vec<Token>,
+    pub body: Vec<Statement>,
+    pub closure: Rc<RefCell<Environment>>,
+}
+
+impl Display for Function {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.name.lexeme)
+    }
+}
+
+impl PartialEq for Function {
+    fn eq(&self, other: &Self) -> bool {
+        self.name == other.name && self.params == other.params && self.body == other.body
+    }
+}
+
+impl Callable for Function {
+    fn name(&self) -> String {
+        self.name.lexeme.clone()
+    }
+
+    fn arity(&self) -> usize {
+        self.params.len()
+    }
+
+    fn call(&self, interpreter: &mut Interpreter, args: Vec<Value>) -> Result<Value, CallingError> {
+        let mut env = Environment::with_enclosing(self.closure.clone());
+        for (i, param) in self.params.iter().enumerate() {
+            let arg = args
+                .get(i)
+                .ok_or(CallingError::ArgumentMismatch(
+                    self.params.len(),
+                    args.len(),
+                ))?
+                .clone();
+            env.define(param.lexeme.clone(), arg);
+        }
+
+        let result = interpreter
+            .block(&self.body, env)
+            .map_err(|e| CallingError::CallInterpretationFailed(e.to_string()))?
+            .unwrap_or(Value::Nil);
+        Ok(result)
+    }
+}
diff --git a/rust/rox/src/interpreter.rs b/rust/rox/src/interpreter.rs
index 1438d81..239d043 100644
--- a/rust/rox/src/interpreter.rs
+++ b/rust/rox/src/interpreter.rs
@@ -4,8 +4,11 @@ use thiserror::Error;
 use tracing::error;
 
 use crate::{
+    callable::CallingError,
     environment::{Environment, EnvironmentError},
     expression::Expression,
+    function::Function,
+    native_functions,
     statement::Statement,
     token::{Literal, Token, TokenType},
     value::Value,
@@ -25,14 +28,39 @@ pub enum InterpreterError {
     BinaryExpressionNeedsNumberOrString(usize),
     #[error("{0}")]
     UndefinedVariable(EnvironmentError),
+    #[error("[line {0}] {1} is not callable.")]
+    NotACallable(usize, Value),
+    #[error("[line {0}] {1}.")]
+    FailedToCall(usize, CallingError),
 }
 
 /// Interpreter for the Lox language.
-#[derive(Default, Debug)]
+#[derive(Debug)]
 pub struct Interpreter {
+    pub globals: Rc<RefCell<Environment>>,
     environment: Rc<RefCell<Environment>>,
 }
 
+/// Default configuration for the interpreter, with builtin native functions.
+impl Default for Interpreter {
+    fn default() -> Self {
+        let env: Rc<RefCell<Environment>> = Default::default();
+
+        {
+            // add all native functions to global environment
+            let mut env = env.borrow_mut();
+            for (name, fun) in native_functions::all() {
+                env.define(name, fun);
+            }
+        }
+
+        Self {
+            globals: env.clone(),
+            environment: env.clone(),
+        }
+    }
+}
+
 impl Interpreter {
     /// Try to evaluate an expression and return its result.
     pub fn run(&mut self, statements: Vec<Statement>) -> Result<(), InterpreterError> {
@@ -47,18 +75,16 @@ impl Interpreter {
     }
 
     ///Execute a statement.
-    fn execute(&mut self, statement: &Statement) -> Result<(), InterpreterError> {
+    fn execute(&mut self, statement: &Statement) -> Result<Option<Value>, InterpreterError> {
         match statement {
             Statement::Block(statements) => {
                 let sub_env = Environment::with_enclosing(self.environment.clone());
-                self.block(statements, sub_env)?
-            }
-            Statement::Print(expression) => self.print_statement(expression)?,
-            Statement::Expression(expression) => {
-                self.evaluate(expression)?;
+                self.block(statements, sub_env)
             }
+            Statement::Print(expression) => self.print_statement(expression),
+            Statement::Expression(expression) => Ok(Some(self.evaluate(expression)?)),
             Statement::Var { name, initializer } => {
-                self.var_statement(name, initializer.as_ref().as_ref())?
+                self.var_statement(name, initializer.as_ref().as_ref())
             }
             Statement::If {
                 condition,
@@ -66,33 +92,43 @@ impl Interpreter {
                 else_branch,
             } => {
                 let else_branch = else_branch.as_ref().map(|x| *x.clone());
-                self.if_statement(condition, then_branch, else_branch.as_ref())?
+                self.if_statement(condition, then_branch, else_branch.as_ref())
             }
-            Statement::While { condition, body } => self.while_statement(condition, body)?,
-        };
-
-        Ok(())
+            Statement::While { condition, body } => self.while_statement(condition, body),
+            Statement::Function { name, params, body } => {
+                self.function_statement(name, params, body)
+            }
+            Statement::Return { keyword: _, value } => self.return_statement(value),
+        }
     }
 
     /// Execute all statements within a block, using a new environment (with the old one as the
-    /// enclosing one).
-    fn block(
+    /// enclosing one). Immediately return the value if a Return Value is encountered after
+    /// execution of a statement.
+    pub fn block(
         &mut self,
-        statements: &Vec<Statement>,
+        statements: &[Statement],
         environment: Environment,
-    ) -> Result<(), InterpreterError> {
+    ) -> Result<Option<Value>, InterpreterError> {
         let prev_env = self.environment.clone();
         self.environment = Rc::new(RefCell::new(environment));
 
         for stmt in statements {
-            if let Err(e) = self.execute(stmt) {
-                error!("{e}");
+            let execution = self.execute(stmt);
+            match execution {
+                Ok(x) => {
+                    if let Some(Value::Return(x)) = x {
+                        self.environment = prev_env.clone();
+                        return Ok(Some(*x));
+                    }
+                }
+                Err(e) => error!("{e}"),
             }
         }
 
         self.environment = prev_env.clone();
 
-        Ok(())
+        Ok(None)
     }
 
     /// Evaluate an expression and return its value.
@@ -116,31 +152,95 @@ impl Interpreter {
                 operator,
                 right,
             } => self.logical_expression(left, operator, right),
+            Expression::Call {
+                callee,
+                paren,
+                args,
+            } => self.call(callee, paren, args),
         }
     }
 
+    /// Call a callable if it is one (meaning it starts with a LeftParen after an identifier),
+    /// otherwise evaluate the expression.
+    fn call(
+        &mut self,
+        callee: &Expression,
+        paren: &Token,
+        arg_expressions: &[Expression],
+    ) -> Result<Value, InterpreterError> {
+        let callee = self.evaluate(callee)?;
+        let mut args = Vec::new();
+
+        for arg in arg_expressions {
+            args.push(self.evaluate(arg)?);
+        }
+
+        if let Value::Callable(f) = callee {
+            f.call(self, args)
+                .map_err(|e| InterpreterError::FailedToCall(paren.line, e))
+        } else {
+            Err(InterpreterError::NotACallable(paren.line, callee))
+        }
+    }
+
+    /// Define a new function.
+    fn function_statement(
+        &mut self,
+        name: &Token,
+        params: &[Token],
+        body: &[Statement],
+    ) -> Result<Option<Value>, InterpreterError> {
+        let fn_name = name.lexeme.clone();
+        let fun = Function {
+            name: name.clone(),
+            params: params.to_vec(),
+            body: body.to_vec(),
+            closure: self.environment.clone(),
+        };
+
+        self.environment
+            .borrow_mut()
+            .define(fn_name, Value::Callable(Rc::new(fun)));
+
+        Ok(None)
+    }
+
     /// If the condition evaluates to truthy, execute the then branch, otherwise the else branch.
     fn if_statement(
         &mut self,
         condition: &Expression,
         then_branch: &Statement,
         else_branch: Option<&Statement>,
-    ) -> Result<(), InterpreterError> {
+    ) -> Result<Option<Value>, InterpreterError> {
         if self.evaluate(condition)?.is_truthy() {
             self.execute(then_branch)
         } else if let Some(else_branch) = else_branch {
             self.execute(else_branch)
         } else {
-            Ok(())
+            Ok(None)
         }
     }
 
     /// Evaluate an expression and print its value to stdout.
-    fn print_statement(&mut self, expression: &Expression) -> Result<(), InterpreterError> {
+    fn print_statement(
+        &mut self,
+        expression: &Expression,
+    ) -> Result<Option<Value>, InterpreterError> {
         let value = self.evaluate(expression)?;
         println!("{value}");
 
-        Ok(())
+        Ok(None)
+    }
+
+    /// Return a value from a statement.
+    fn return_statement(
+        &mut self,
+        value: &Option<Expression>,
+    ) -> Result<Option<Value>, InterpreterError> {
+        Ok(match value {
+            Some(x) => Some(Value::Return(Box::new(self.evaluate(x)?))),
+            None => Some(Value::Return(Box::new(Value::Nil))),
+        })
     }
 
     /// Initialize a variable with an initializer expression or nil.
@@ -148,7 +248,7 @@ impl Interpreter {
         &mut self,
         name: &Token,
         initializer: Option<&Expression>,
-    ) -> Result<(), InterpreterError> {
+    ) -> Result<Option<Value>, InterpreterError> {
         let value = if let Some(initializer) = initializer {
             self.evaluate(initializer)
         } else {
@@ -159,7 +259,7 @@ impl Interpreter {
             .borrow_mut()
             .define(name.lexeme.clone(), value);
 
-        Ok(())
+        Ok(None)
     }
 
     /// Execute the body as long as the condition evaluates to true.
@@ -167,12 +267,12 @@ impl Interpreter {
         &mut self,
         condition: &Expression,
         body: &Statement,
-    ) -> Result<(), InterpreterError> {
+    ) -> Result<Option<Value>, InterpreterError> {
         while self.evaluate(condition)?.is_truthy() {
             self.execute(body)?;
         }
 
-        Ok(())
+        Ok(None)
     }
 
     /// Assign the value of an expression to a variable.
diff --git a/rust/rox/src/lib.rs b/rust/rox/src/lib.rs
index dd37957..81757f9 100644
--- a/rust/rox/src/lib.rs
+++ b/rust/rox/src/lib.rs
@@ -10,11 +10,14 @@ use std::{
 use interpreter::Interpreter;
 use tracing::error;
 
+pub mod callable;
 pub mod cli;
 pub mod environment;
 pub mod expression;
+pub mod function;
 pub mod interpreter;
 pub mod keywords;
+pub mod native_functions;
 pub mod parser;
 pub mod scanner;
 pub mod statement;
diff --git a/rust/rox/src/native_functions.rs b/rust/rox/src/native_functions.rs
new file mode 100644
index 0000000..1268b41
--- /dev/null
+++ b/rust/rox/src/native_functions.rs
@@ -0,0 +1,39 @@
+use std::{
+    rc::Rc,
+    time::{SystemTime, UNIX_EPOCH},
+};
+
+use crate::{
+    callable::{Callable, CallingError},
+    interpreter::Interpreter,
+    value::Value,
+};
+
+struct Clock;
+
+impl Callable for Clock {
+    fn name(&self) -> String {
+        "clock".into()
+    }
+
+    fn arity(&self) -> usize {
+        0
+    }
+
+    fn call(
+        &self,
+        _interpreter: &mut Interpreter,
+        _args: Vec<crate::value::Value>,
+    ) -> Result<Value, CallingError> {
+        let start = SystemTime::now();
+        let since_the_epoch = start
+            .duration_since(UNIX_EPOCH)
+            .map_err(|e| CallingError::CallFailed(e.to_string()))?;
+
+        Ok(Value::Number(since_the_epoch.as_secs_f64()))
+    }
+}
+
+pub fn all() -> Vec<(String, Value)> {
+    vec![("clock".into(), Value::Callable(Rc::new(Clock {})))]
+}
diff --git a/rust/rox/src/parser.rs b/rust/rox/src/parser.rs
index faab052..1d0892c 100644
--- a/rust/rox/src/parser.rs
+++ b/rust/rox/src/parser.rs
@@ -1,6 +1,3 @@
-use thiserror::Error;
-use tracing::error;
-
 use crate::{
     expression::Expression,
     statement::Statement,
@@ -9,6 +6,8 @@ use crate::{
         TokenType::{self, *},
     },
 };
+use thiserror::Error;
+use tracing::error;
 
 #[derive(Error, Debug)]
 pub enum ParserError {
@@ -44,6 +43,20 @@ pub enum ParserError {
     SemicolonAfterLoopConditionExpected(usize),
     #[error("[line {0}] expected ')' after for clauses.")]
     RightParenAfterForClausesExpected(usize),
+    #[error("[line {0}] expected ')' after arguments.")]
+    RightParenAfterArgumentsExpected(usize),
+    #[error("[line {0}] expected function name.")]
+    FunctionNameExpected(usize),
+    #[error("[line {0}] expected '(' after function name.")]
+    LeftParenAfterFunctionNameExpected(usize),
+    #[error("[line {0}] expected ')' after parameters.")]
+    RightParenAfterParamsExpected(usize),
+    #[error("[line {0}] expected parameter name.")]
+    ParamNameExpected(usize),
+    #[error("[line {0}] expected '{{' before function body.")]
+    LeftBraceBeforeFunctionBodyExpected(usize),
+    #[error("[line {0}] expected ';' after return value.")]
+    SemicolonAfterReturnExpected(usize),
 }
 
 /// Parse the Lox language.
@@ -161,7 +174,9 @@ impl Parser {
     }
 
     fn declaration(&mut self) -> Result<Statement, ParserError> {
-        if self.matches(&[Var]) {
+        if self.matches(&[Fun]) {
+            self.function()
+        } else if self.matches(&[Var]) {
             self.var_declaration()
         } else {
             self.statement()
@@ -175,6 +190,8 @@ impl Parser {
             self.if_statement()
         } else if self.matches(&[Print]) {
             self.print_statement()
+        } else if self.matches(&[Return]) {
+            self.return_statement()
         } else if self.matches(&[While]) {
             self.while_statement()
         } else if self.matches(&[LeftBrace]) {
@@ -266,6 +283,20 @@ impl Parser {
         Ok(Statement::Print(value))
     }
 
+    fn return_statement(&mut self) -> Result<Statement, ParserError> {
+        let keyword = self.previous()?.clone();
+        let value = if self.check(&Semicolon) {
+            None
+        } else {
+            Some(self.expression()?)
+        };
+
+        self.consume(&Semicolon)
+            .ok_or(ParserError::SemicolonAfterReturnExpected(keyword.line))?;
+
+        Ok(Statement::Return { keyword, value })
+    }
+
     fn var_declaration(&mut self) -> Result<Statement, ParserError> {
         let line = self.current_token.line;
         let name = self
@@ -315,6 +346,43 @@ impl Parser {
         Ok(Statement::Expression(expr))
     }
 
+    fn function(&mut self) -> Result<Statement, ParserError> {
+        let line = self.current_token.line;
+        let name = self
+            .consume(&Identifier)
+            .ok_or(ParserError::FunctionNameExpected(line))?
+            .clone();
+
+        self.consume(&LeftParen)
+            .ok_or(ParserError::LeftParenAfterFunctionNameExpected(line))?;
+
+        let mut params = Vec::new();
+        if !self.check(&RightParen) {
+            let param = self
+                .consume(&Identifier)
+                .ok_or(ParserError::ParamNameExpected(line))?
+                .clone();
+            params.push(param);
+
+            while self.matches(&[Comma]) {
+                let param = self
+                    .consume(&Identifier)
+                    .ok_or(ParserError::ParamNameExpected(line))?
+                    .clone();
+                params.push(param);
+            }
+        }
+
+        self.consume(&RightParen)
+            .ok_or(ParserError::RightParenAfterParamsExpected(line))?;
+        self.consume(&LeftBrace)
+            .ok_or(ParserError::LeftBraceBeforeFunctionBodyExpected(line))?;
+
+        let body = self.block()?;
+
+        Ok(Statement::Function { name, params, body })
+    }
+
     fn block(&mut self) -> Result<Vec<Statement>, ParserError> {
         let mut statements = Vec::new();
 
@@ -412,10 +480,48 @@ impl Parser {
                 right: Box::new(right),
             })
         } else {
-            self.primary()
+            self.call()
         }
     }
 
+    fn call(&mut self) -> Result<Expression, ParserError> {
+        let mut expr = self.primary()?;
+
+        loop {
+            if self.matches(&[LeftParen]) {
+                expr = self.finish_call(expr)?;
+            } else {
+                break;
+            }
+        }
+
+        Ok(expr)
+    }
+
+    fn finish_call(&mut self, callee: Expression) -> Result<Expression, ParserError> {
+        let mut args = Vec::new();
+
+        if !self.check(&RightParen) {
+            args.push(self.expression()?);
+
+            while self.matches(&[Comma]) {
+                args.push(self.expression()?);
+            }
+        }
+
+        let line = self.current_token.line;
+        let paren = self
+            .consume(&RightParen)
+            .ok_or(ParserError::RightParenAfterArgumentsExpected(line))?
+            .clone();
+
+        Ok(Expression::Call {
+            callee: Box::new(callee),
+            paren,
+            args,
+        })
+    }
+
     /// primary -> NUMBER | STRING | "true" | "false" | "nil" | "(" expression ")" ;
     fn primary(&mut self) -> Result<Expression, ParserError> {
         if self.matches(&[False]) {
diff --git a/rust/rox/src/statement.rs b/rust/rox/src/statement.rs
index 1ade330..c75b3c4 100644
--- a/rust/rox/src/statement.rs
+++ b/rust/rox/src/statement.rs
@@ -19,4 +19,13 @@ pub enum Statement {
         condition: Expression,
         body: Box<Statement>,
     },
+    Function {
+        name: Token,
+        params: Vec<Token>,
+        body: Vec<Statement>,
+    },
+    Return {
+        keyword: Token,
+        value: Option<Expression>,
+    },
 }
diff --git a/rust/rox/src/value.rs b/rust/rox/src/value.rs
index 5efcc64..b272014 100644
--- a/rust/rox/src/value.rs
+++ b/rust/rox/src/value.rs
@@ -1,10 +1,12 @@
-use std::fmt::Display;
+use std::{fmt::Display, rc::Rc};
 
-use crate::token::Literal;
+use crate::{callable::Callable, token::Literal};
 
 /// Concrete value in the interpreter.
 #[derive(Clone, Debug, PartialEq)]
 pub enum Value {
+    Return(Box<Value>),
+    Callable(Rc<dyn Callable>),
     String(String),
     Number(f64),
     Boolean(bool),
@@ -14,6 +16,8 @@ pub enum Value {
 impl Display for Value {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
+            Value::Return(x) => write!(f, "{x}"),
+            Value::Callable(x) => write!(f, "{x}"),
             Value::String(x) => write!(f, "{x}"),
             Value::Number(x) => write!(f, "{x}"),
             Value::Boolean(x) => write!(f, "{x}"),