diff --git a/src/com/lox/Interpreter.java b/src/com/lox/Interpreter.java
new file mode 100644
index 0000000..09a93da
--- /dev/null
+++ b/src/com/lox/Interpreter.java
@@ -0,0 +1,118 @@
+package com.lox;
+
+class Interpreter implements Expr.Visitor<Object> {
+  @Override
+  public Object visitLiteralExpr(Expr.Literal expr) {
+    return expr.value;
+  }
+  @Override
+  public Object visitUnaryExpr(Expr.Unary expr) {
+    Object right = evaluate(expr.right);
+
+    switch (expr.operator.type) {
+      case BANG:
+        return !isTruthy(right);
+      case MINUS:
+        checkNumberOperand(expr.operator, right);
+        return -(double)right;
+    }
+
+    // Unreachable.
+    return null;
+  }
+  private void checkNumberOperand(Token operator, Object operand) {
+    if (operand instanceof Double) return;
+    throw new RuntimeError(operator, "Operand must be a number.");
+  }
+  private void checkNumberOperands(Token operator,
+                                   Object left, Object right) {
+    if (left instanceof Double && right instanceof Double) return;
+    
+    throw new RuntimeError(operator, "Operands must be numbers.");
+  }
+
+  @Override
+  public Object visitGroupingExpr(Expr.Grouping expr) {
+    return evaluate(expr.expression);
+  }
+  @Override
+  public Object visitBinaryExpr(Expr.Binary expr) {
+    Object left = evaluate(expr.left);
+    Object right = evaluate(expr.right); 
+
+    switch (expr.operator.type) {
+      case GREATER:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left > (double)right;
+      case GREATER_EQUAL:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left >= (double)right;
+      case LESS:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left < (double)right;
+      case LESS_EQUAL:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left <= (double)right;
+      case MINUS:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left - (double)right;
+      case PLUS:
+        if (left instanceof Double && right instanceof Double) {
+          return (double)left + (double)right;
+        } 
+
+        if (left instanceof String && right instanceof String) {
+          return (String)left + (String)right;
+        }
+        throw new RuntimeError(expr.operator,
+            "Operands must be two numbers or two strings.");
+      case SLASH:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left / (double)right;
+      case STAR:
+        checkNumberOperands(expr.operator, left, right);
+        return (double)left * (double)right;
+      case BANG_EQUAL: return !isEqual(left, right);
+      case EQUAL_EQUAL: return isEqual(left, right);
+    }
+
+    // Unreachable.
+    return null;
+  }
+  private Object evaluate(Expr expr) {
+    return expr.accept(this);
+  }
+  
+  private boolean isTruthy(Object object) {
+    if (object == null) return false;
+    if (object instanceof Boolean) return (boolean)object;
+    return true;
+  }
+  private boolean isEqual(Object a, Object b) {
+    if (a == null && b == null) return true;
+    if (a == null) return false;
+
+    return a.equals(b);
+  }
+  void interpret(Expr expression) { 
+    try {
+      Object value = evaluate(expression);
+      System.out.println(stringify(value));
+    } catch (RuntimeError error) {
+      Lox.runtimeError(error);
+    }
+  }
+  private String stringify(Object object) {
+    if (object == null) return "nil";
+
+    if (object instanceof Double) {
+      String text = object.toString();
+      if (text.endsWith(".0")) {
+        text = text.substring(0, text.length() - 2);
+      }
+      return text;
+    }
+
+    return object.toString();
+  }
+}
\ No newline at end of file
diff --git a/src/com/lox/Lox.java b/src/com/lox/Lox.java
index b3af507..65c6df8 100644
--- a/src/com/lox/Lox.java
+++ b/src/com/lox/Lox.java
@@ -9,7 +9,9 @@ import java.nio.file.Paths;
 import java.util.List;
 
 public class Lox {
+  private static final Interpreter interpreter = new Interpreter();
   static boolean hadError = false;
+  static boolean hadRuntimeError = false;  
   public static void main(String[] args) throws IOException {
     if (args.length > 1) {
       System.out.println("Usage: jlox [script]");
@@ -29,8 +31,7 @@ public class Lox {
 
     // Stop if there was a syntax error.
     if (hadError) return;
-
-    System.out.println(new AstPrinter().print(expression));
+    interpreter.interpret(expression);    
   }
   static void error(int line, String message) {
     report(line, "", message);
@@ -51,11 +52,17 @@ public class Lox {
     }
   }
 
+  static void runtimeError(RuntimeError error) {
+    System.err.println(error.getMessage() +
+        "\n[line " + error.token.line + "]");
+    hadRuntimeError = true;
+  }
 
   private static void runFile(String path) throws IOException {
     byte[] bytes = Files.readAllBytes(Paths.get(path));
     run(new String(bytes, Charset.defaultCharset()));
     if(hadError) System.exit(65);
+    if(hadRuntimeError) System.exit(70);
   }
   private static void runPrompt() throws IOException {
     InputStreamReader input = new InputStreamReader(System.in);
diff --git a/src/com/lox/RuntimeError.java b/src/com/lox/RuntimeError.java
new file mode 100644
index 0000000..8f737b7
--- /dev/null
+++ b/src/com/lox/RuntimeError.java
@@ -0,0 +1,10 @@
+package com.lox;
+
+class RuntimeError extends RuntimeException {
+  final Token token;
+
+  RuntimeError(Token token, String message) {
+    super(message);
+    this.token = token;
+  }
+}
\ No newline at end of file