diff --git a/jlox/lox/src/main/java/xyz/ctsk/lox/Interpreter.java b/jlox/lox/src/main/java/xyz/ctsk/lox/Interpreter.java
new file mode 100644
index 0000000..176877b
--- /dev/null
+++ b/jlox/lox/src/main/java/xyz/ctsk/lox/Interpreter.java
@@ -0,0 +1,108 @@
+package xyz.ctsk.lox;
+
+public class Interpreter implements Expr.Visitor<Object> {
+
+    void interpret(Expr expression) {
+        try {
+            Object value = evaluate(expression);
+            System.out.println(stringify(value));
+        } catch (RuntimeError error) {
+            Lox.runtimeError(error);
+        }
+    }
+
+    private Object evaluate(Expr expr) {
+        return expr.accept(this);
+    }
+
+    private String stringify(Object object) {
+        if (object == null) return "nil";
+
+        var text = object.toString();
+
+        if (object instanceof Double && text.endsWith(".0")) {
+            text = text.substring(0, text.length() - 2);
+        }
+        return text;
+    }
+
+
+    @Override
+    public Object visitBinaryExpr(Expr.Binary binary) {
+        var left = evaluate(binary.left());
+        var right = evaluate(binary.right());
+
+        switch (binary.operator().type()) {
+            case MINUS, SLASH, STAR, GREATER, GREATER_EQUAL, LESS, LESS_EQUAL ->
+                    checkNumberOperands(binary.operator(), left, right);
+        }
+
+        return switch (binary.operator().type()) {
+            case MINUS -> (double) left - (double) right;
+            case PLUS  -> {
+                if (left instanceof Double leftD && right instanceof Double rightD) {
+                    yield leftD + rightD;
+                }
+
+                if (left instanceof String leftStr && right instanceof String rightStr) {
+                    yield leftStr + rightStr;
+                }
+
+                throw new RuntimeError(binary.operator(), "Operands must be two numbers or two strings.");
+            }
+            case SLASH -> (double) left / (double) right;
+            case STAR  -> (double) left * (double) right;
+            case GREATER       -> (double)left > (double)right;
+            case GREATER_EQUAL -> (double)left >= (double)right;
+            case LESS          -> (double)left < (double)right;
+            case LESS_EQUAL    -> (double)left <= (double)right;
+            case BANG_EQUAL  -> !isEqual(left, right);
+            case EQUAL_EQUAL -> isEqual(left, right);
+            default -> null;
+        };
+    }
+
+    @Override
+    public Object visitGroupingExpr(Expr.Grouping grouping) {
+        return evaluate(grouping.expression());
+    }
+
+    @Override
+    public Object visitLiteralExpr(Expr.Literal literal) {
+        return literal.value();
+    }
+
+    @Override
+    public Object visitUnaryExpr(Expr.Unary unary) {
+        var right = evaluate(unary.right());
+
+        return switch(unary.operator().type()) {
+            case MINUS -> -asNumber(unary.operator(), right);
+            case BANG -> !isTruthy(right);
+            default -> null;
+        };
+    }
+
+
+    private boolean isTruthy(Object object) {
+        if (object == null) return false;
+        if (object instanceof Boolean bool) return bool;
+        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);
+    }
+
+    private double asNumber(Token operator, Object operand) {
+        if (operand instanceof Double d) return d;
+        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");
+    }
+}
diff --git a/jlox/lox/src/main/java/xyz/ctsk/lox/Lox.java b/jlox/lox/src/main/java/xyz/ctsk/lox/Lox.java
index e130870..3c43e75 100644
--- a/jlox/lox/src/main/java/xyz/ctsk/lox/Lox.java
+++ b/jlox/lox/src/main/java/xyz/ctsk/lox/Lox.java
@@ -8,7 +8,10 @@ import java.nio.file.Files;
 import java.nio.file.Paths;
 
 public class Lox {
+    private static final Interpreter interpreter = new Interpreter();
+
     private static boolean hadError = false;
+    private static boolean hadRuntimeError = false;
 
     private static void report(int line, String where, String message) {
          System.err.printf("[line %d] Error %s: %s%n", line, where, message);
@@ -27,6 +30,11 @@ public class Lox {
         }
     }
 
+    static void runtimeError(RuntimeError error) {
+        System.err.printf("%s%n[line %d]%n", error.getMessage(), error.token.line());
+        hadRuntimeError = true;
+    }
+
     private static void run(String source) {
         var scanner = new Scanner(source);
         var tokens = scanner.scanTokens();
@@ -36,8 +44,7 @@ public class Lox {
 
         if (hadError) return;
 
-        System.out.println(AstPrinter.polish(expression, true));
-        System.out.println(AstPrinter.reverse_polish(expression, false));
+        interpreter.interpret(expression);
     }
 
     private static void runPrompt() throws IOException {
@@ -57,9 +64,8 @@ public class Lox {
         byte[] bytes = Files.readAllBytes(Paths.get(path));
         run(new String(bytes, Charset.defaultCharset()));
 
-        if (hadError) {
-            System.exit(65);
-        }
+        if (hadError) System.exit(65);
+        if (hadRuntimeError) System.exit(70);
     }
 
     private static void printUsage() {
diff --git a/jlox/lox/src/main/java/xyz/ctsk/lox/RuntimeError.java b/jlox/lox/src/main/java/xyz/ctsk/lox/RuntimeError.java
new file mode 100644
index 0000000..d421e33
--- /dev/null
+++ b/jlox/lox/src/main/java/xyz/ctsk/lox/RuntimeError.java
@@ -0,0 +1,10 @@
+package xyz.ctsk.lox;
+
+public class RuntimeError extends RuntimeException {
+    final Token token;
+
+    RuntimeError(Token token, String message) {
+        super(message);
+        this.token = token;
+    }
+}