diff --git a/src/lox/ASTPrinter.java b/src/lox/ASTPrinter.java
index 4d853c4..18587e8 100644
--- a/src/lox/ASTPrinter.java
+++ b/src/lox/ASTPrinter.java
@@ -28,6 +28,19 @@ class ASTPrinter implements Expr.Visitor<String> {
     return parenthesize(expr.operator.lexeme, expr.right);
   }
 
+  @Override
+  public String visitCallExpr(Expr.Call expr) {
+    StringBuilder builder = new StringBuilder();
+
+    builder.append(expr.callee.toString()).append("(");
+    for (Expr arg : expr.arguments) {
+      builder.append(arg.accept(this)).append(", ");
+    }
+    builder.append(")");
+
+    return builder.toString();
+  }
+
   @Override
   public String visitVariableExpr(Expr.Variable expr) {
     return expr.name.toString();
diff --git a/src/lox/Expr.java b/src/lox/Expr.java
index 65a7b97..5c0d816 100644
--- a/src/lox/Expr.java
+++ b/src/lox/Expr.java
@@ -6,6 +6,7 @@ abstract class Expr {
   interface Visitor<R> {
     R visitAssignExpr(Assign expr);
     R visitBinaryExpr(Binary expr);
+    R visitCallExpr(Call expr);
     R visitGroupingExpr(Grouping expr);
     R visitLiteralExpr(Literal expr);
     R visitLogicalExpr(Logical expr);
@@ -45,6 +46,23 @@ abstract class Expr {
     final Expr right;
   }
 
+  static class Call extends Expr {
+    Call(Expr callee, Token paren, List<Expr> arguments) {
+      this.callee = callee;
+      this.paren = paren;
+      this.arguments = arguments;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitCallExpr(this);
+    }
+
+    final Expr callee;
+    final Token paren;
+    final List<Expr> arguments;
+  }
+
   static class Grouping extends Expr {
     Grouping(Expr expression) {
       this.expression = expression;
diff --git a/src/lox/Interpreter.java b/src/lox/Interpreter.java
index aa6609e..6c203f8 100644
--- a/src/lox/Interpreter.java
+++ b/src/lox/Interpreter.java
@@ -1,9 +1,32 @@
 package lox;
 
+import java.util.ArrayList;
 import java.util.List;
 
 class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
-  private Environment environment = new Environment();
+  final Environment globals = new Environment();
+  private Environment environment = globals;
+
+  Interpreter() {
+    globals.define(
+        "clock",
+        new LoxCallable() {
+          @Override
+          public int arity() {
+            return 0;
+          }
+
+          @Override
+          public Object call(Interpreter interpreter, List<Object> arguments) {
+            return (double) System.currentTimeMillis() / 1000.0;
+          }
+
+          @Override
+          public String toString() {
+            return "<native fn>";
+          }
+        });
+  }
 
   void interpret(List<Stmt> statements) {
     try {
@@ -57,6 +80,27 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
     return null;
   }
 
+  @Override
+  public Object visitCallExpr(Expr.Call expr) {
+    Object callee = evaluate(expr.callee);
+
+    List<Object> arguments = new ArrayList<>();
+    for (Expr argument : expr.arguments) {
+      arguments.add(evaluate(argument));
+    }
+
+    if (!(callee instanceof LoxCallable)) {
+      throw new RuntimeError(expr.paren, "Can only call functions and classes");
+    }
+    LoxCallable function = (LoxCallable) callee;
+    if (arguments.size() != function.arity()) {
+      throw new RuntimeError(
+          expr.paren,
+          "Expected " + function.arity() + " arguments but called with " + arguments.size() + ".");
+    }
+    return function.call(this, arguments);
+  }
+
   @Override
   public Object visitBinaryExpr(Expr.Binary expr) {
     Object left = evaluate(expr.left);
@@ -119,6 +163,13 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
     return null;
   }
 
+  @Override
+  public Void visitFunctionStmt(Stmt.Function stmt) {
+    LoxFunction function = new LoxFunction(stmt, environment);
+    environment.define(stmt.name.lexeme, function);
+    return null;
+  }
+
   @Override
   public Void visitIfStmt(Stmt.If stmt) {
     if (isTruthy(evaluate(stmt.condition))) {
@@ -136,6 +187,15 @@ class Interpreter implements Expr.Visitor<Object>, Stmt.Visitor<Void> {
     return null;
   }
 
+  @Override
+  public Void visitReturnStmt(Stmt.Return stmt) {
+    Object value = null;
+    if (stmt.value != null) {
+      value = evaluate(stmt.value);
+    }
+    throw new Return(value);
+  }
+
   @Override
   public Void visitVarStmt(Stmt.Var stmt) {
     Object value = null;
diff --git a/src/lox/LoxCallable.java b/src/lox/LoxCallable.java
new file mode 100644
index 0000000..f413593
--- /dev/null
+++ b/src/lox/LoxCallable.java
@@ -0,0 +1,9 @@
+package lox;
+
+import java.util.List;
+
+interface LoxCallable {
+  int arity();
+
+  Object call(Interpreter interpreter, List<Object> arguments);
+}
diff --git a/src/lox/LoxFunction.java b/src/lox/LoxFunction.java
new file mode 100644
index 0000000..4b188ad
--- /dev/null
+++ b/src/lox/LoxFunction.java
@@ -0,0 +1,37 @@
+package lox;
+
+import java.util.List;
+
+class LoxFunction implements LoxCallable {
+  private final Stmt.Function declaration;
+  private final Environment closure;
+
+  LoxFunction(Stmt.Function declaration, Environment closure) {
+    this.closure = closure;
+    this.declaration = declaration;
+  }
+
+  @Override
+  public Object call(Interpreter interpreter, List<Object> arguments) {
+    Environment environment = new Environment(closure);
+    for (int i = 0; i < declaration.params.size(); i++) {
+      environment.define(declaration.params.get(i).lexeme, arguments.get(i));
+    }
+    try {
+      interpreter.executeBlock(declaration.body, environment);
+    } catch (Return returnValue) {
+      return returnValue.value;
+    }
+    return null;
+  }
+
+  @Override
+  public int arity() {
+    return declaration.params.size();
+  }
+
+  @Override
+  public String toString() {
+    return "<fn " + declaration.name.lexeme + ">";
+  }
+}
diff --git a/src/lox/Parser.java b/src/lox/Parser.java
index 154a5b7..1d7e773 100644
--- a/src/lox/Parser.java
+++ b/src/lox/Parser.java
@@ -26,6 +26,9 @@ class Parser {
 
   private Stmt declaration() {
     try {
+      if (match(FUN)) {
+        return function("function");
+      }
       if (match(VAR)) {
         return varDeclaration();
       }
@@ -57,6 +60,9 @@ class Parser {
     if (match(PRINT)) {
       return printStatement();
     }
+    if (match(RETURN)) {
+      return returnStatement();
+    }
     if (match(WHILE)) {
       return whileStatement();
     }
@@ -72,6 +78,16 @@ class Parser {
     return new Stmt.Print(value);
   }
 
+  private Stmt returnStatement() {
+    Token keyword = previous();
+    Expr value = null;
+    if (!check(SEMICOLON)) {
+      value = expression();
+    }
+    consume(SEMICOLON, "Expect ';' after return value");
+    return new Stmt.Return(keyword, value);
+  }
+
   private Stmt whileStatement() {
     consume(LEFT_PAREN, "Expect '(' after 'while");
     Expr condition = expression();
@@ -87,6 +103,24 @@ class Parser {
     return new Stmt.Expression(value);
   }
 
+  private Stmt function(String kind) {
+    Token name = consume(IDENTIFIER, "Expect " + kind + " name.");
+    consume(LEFT_PAREN, "Expect '(' after " + kind + "name");
+    List<Token> parameters = new ArrayList<>();
+    if (!check(RIGHT_PAREN)) {
+      do {
+        if (parameters.size() >= 255) {
+          error(peek(), "Can't have more than 255 parameters.");
+        }
+        parameters.add(consume(IDENTIFIER, "Expect parameter name."));
+      } while (match(COMMA));
+    }
+    consume(RIGHT_PAREN, "Expect ')' after parameters.");
+    consume(LEFT_BRACE, "Expect '{' before " + kind + " body.");
+    List<Stmt> body = block();
+    return new Stmt.Function(name, parameters, body);
+  }
+
   private Stmt ifStatement() {
     consume(LEFT_PAREN, "Expect '(' after 'if'.");
     Expr condition = expression();
@@ -245,7 +279,38 @@ class Parser {
       Expr right = unary();
       return new Expr.Unary(operator, right);
     }
-    return primary();
+    return call();
+  }
+
+  private Expr call() {
+    Expr expr = primary();
+
+    while (true) {
+      if (match(LEFT_PAREN)) {
+        expr = finishCall(expr);
+      } else {
+        break;
+      }
+    }
+
+    return expr;
+  }
+
+  private Expr finishCall(Expr callee) {
+    List<Expr> arguments  = new ArrayList<>();
+
+    if (!check(RIGHT_PAREN)) {
+      do {
+        arguments.add(expression());
+      } while (match(COMMA));
+    }
+    Token paren = consume(RIGHT_PAREN, "Expected ')' after argument list");
+
+    if (arguments.size() >= 255) {
+      error(peek(), "Can't have more than 255 arguments");
+    }
+
+    return new Expr.Call(callee, paren, arguments);
   }
 
   private Expr primary() {
diff --git a/src/lox/Return.java b/src/lox/Return.java
new file mode 100644
index 0000000..d8338fb
--- /dev/null
+++ b/src/lox/Return.java
@@ -0,0 +1,10 @@
+package lox;
+
+class Return extends RuntimeException{
+  final Object value;
+
+  Return(Object value) {
+    super(null, null, false, false);
+    this.value = value;
+  }
+}
diff --git a/src/lox/Stmt.java b/src/lox/Stmt.java
index f097da1..3d0b5ac 100644
--- a/src/lox/Stmt.java
+++ b/src/lox/Stmt.java
@@ -6,9 +6,11 @@ abstract class Stmt {
   interface Visitor<R> {
     R visitBlockStmt(Block stmt);
     R visitExpressionStmt(Expression stmt);
+    R visitFunctionStmt(Function stmt);
     R visitIfStmt(If stmt);
     R visitVarStmt(Var stmt);
     R visitPrintStmt(Print stmt);
+    R visitReturnStmt(Return stmt);
     R visitWhileStmt(While stmt);
   }
 
@@ -38,6 +40,23 @@ abstract class Stmt {
     final Expr expression;
   }
 
+  static class Function extends Stmt {
+    Function(Token name, List<Token> params, List<Stmt> body) {
+      this.name = name;
+      this.params = params;
+      this.body = body;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitFunctionStmt(this);
+    }
+
+    final Token name;
+    final List<Token> params;
+    final List<Stmt> body;
+  }
+
   static class If extends Stmt {
     If(Expr condition, Stmt thenBranch, Stmt elseBranch) {
       this.condition = condition;
@@ -83,6 +102,21 @@ abstract class Stmt {
     final Expr expression;
   }
 
+  static class Return extends Stmt {
+    Return(Token keyword, Expr value) {
+      this.keyword = keyword;
+      this.value = value;
+    }
+
+    @Override
+    <R> R accept(Visitor<R> visitor) {
+      return visitor.visitReturnStmt(this);
+    }
+
+    final Token keyword;
+    final Expr value;
+  }
+
   static class While extends Stmt {
     While(Expr condition, Stmt body) {
       this.condition = condition;
diff --git a/src/tool/GenerateAST.java b/src/tool/GenerateAST.java
index b91e423..dc127c2 100644
--- a/src/tool/GenerateAST.java
+++ b/src/tool/GenerateAST.java
@@ -20,6 +20,7 @@ public class GenerateAST {
         Arrays.asList(
             "Assign   : Token name, Expr value",
             "Binary   : Expr left, Token operator, Expr right",
+            "Call     : Expr callee, Token paren, List<Expr> arguments",
             "Grouping : Expr expression",
             "Literal  : Object value",
             "Logical  : Expr left, Token operator, Expr right",
@@ -32,9 +33,11 @@ public class GenerateAST {
         Arrays.asList(
             "Block      : List<Stmt> statements",
             "Expression : Expr expression",
+            "Function   : Token name, List<Token> params, List<Stmt> body",
             "If         : Expr condition, Stmt thenBranch, Stmt elseBranch",
             "Var        : Token name, Expr initializer",
             "Print      : Expr expression",
+            "Return     : Token keyword, Expr value",
             "While      : Expr condition, Stmt body"));
   }