diff --git a/Interpreter.cs b/Interpreter.cs
new file mode 100644
index 0000000..987f9c5
--- /dev/null
+++ b/Interpreter.cs
@@ -0,0 +1,289 @@
+using System;
+using System.Collections.Immutable;
+
+using AST = Finn.AST;
+
+namespace Finn;
+
+public class Interpreter : AST.IExprVisitor<object>
+{
+	public class RuntimeError : Exception
+	{
+		public readonly Token Token;
+
+		internal RuntimeError(Token token, String message) : base(message)
+		{
+			Token = token;
+		}
+	}
+
+	private static void checkTypesEqual(object a, object b)
+	{
+		var aType = a.GetType();
+		var bType = b.GetType();
+		if (aType != bType)
+		{
+			throw new Exception("Type mismatch: {aType} != {bType}.");
+		}
+	}
+
+	private class Record
+	{
+		public static Record Empty = new Record
+		{
+			Fields = ImmutableDictionary<string, ImmutableStack<object>>.Empty,
+		};
+		public required ImmutableDictionary<string, ImmutableStack<object>> Fields { get; init; }
+
+		public Record Update(string name, object value)
+		{
+			ImmutableStack<object>? values;
+			if (!Fields.TryGetValue(name, out values))
+			{
+				throw new ArgumentException($"no such field: {name}");
+			}
+			checkTypesEqual(value, values.Peek());
+			return new Record { Fields = Fields.SetItem(name, values!.Pop().Push(value)) };
+		}
+
+		public object Get(string name)
+		{
+			ImmutableStack<object>? values;
+			if (!Fields.TryGetValue(name, out values))
+			{
+				throw new ArgumentException($"no such field: {name}");
+			}
+			return values.Peek();
+		}
+
+		public Record Remove(string name)
+		{
+			ImmutableStack<object>? values;
+			if (!Fields.TryGetValue(name, out values))
+			{
+				throw new ArgumentException($"no such field: {name}");
+			}
+			var value = values.Peek();
+			var popped = values.Pop();
+			if (popped.IsEmpty)
+			{
+				return new Record { Fields = Fields.Remove(name) };
+			}
+			return new Record { Fields = Fields.SetItem(name, popped) };
+		}
+
+		public Record Extend(string name, object value)
+		{
+			var values = Fields.GetValueOrDefault(name, ImmutableStack<object>.Empty);
+			return new Record { Fields = Fields.SetItem(name, values.Push(value)) };
+		}
+	}
+
+	private record Variant(string Tag, object? Value)
+	{
+		public static readonly Variant True = new Variant("true", null);
+		public static readonly Variant False = new Variant("false", null);
+		public static Variant FromBool(bool b)
+		{
+			return b ? True : False;
+		}
+		public bool IsEmpty { get { return Value == null; } }
+
+		public override string ToString()
+		{
+			if (Value == null)
+			{
+				return $"`{Tag}";
+			}
+			else
+			{
+				return $"`{Tag}({Value})";
+			}
+		}
+	}
+
+	public void Interpret(AST.Expr expression)
+	{
+		try
+		{
+			var value = evaluate(expression);
+			Console.WriteLine(value);
+		}
+		catch (RuntimeError err)
+		{
+			Program.runtimeError(err);
+		}
+	}
+
+	private object evaluate(AST.Expr expr)
+	{
+		return expr.accept(this);
+	}
+
+	private void checkNumberOperand(Token op, Object operand)
+	{
+		if (operand is double) return;
+		throw new RuntimeError(op, "Operand must be a number.");
+	}
+
+	private void checkNumberOperands(Token op, object left, object right)
+	{
+		if (left is double && right is double) return;
+		throw new RuntimeError(op, "Operands must be numbers.");
+	}
+
+	private void checkStringOperands(Token op, object left, object right)
+	{
+		if (left is string && right is string) return;
+		throw new RuntimeError(op, "Operands must be strings.");
+	}
+
+	public object visitBinaryExpr(AST.Binary expr)
+	{
+		var left = evaluate(expr.Left);
+		var right = evaluate(expr.Right);
+
+		switch (expr.Op.type)
+		{
+			case TokenType.Minus:
+				checkNumberOperands(expr.Op, left, right);
+				return (double)left - (double)right;
+			case TokenType.Plus:
+				checkNumberOperands(expr.Op, left, right);
+				return (double)left + (double)right;
+			case TokenType.Slash:
+				checkNumberOperands(expr.Op, left, right);
+				return (double)left / (double)right;
+			case TokenType.Asterisk:
+				checkNumberOperands(expr.Op, left, right);
+				return (double)left * (double)right;
+
+			case TokenType.PlusPlus:
+				checkStringOperands(expr.Op, left, right);
+				return (string)left + (string)right;
+
+			case TokenType.Greater:
+				checkNumberOperands(expr.Op, left, right);
+				return Variant.FromBool((double)left > (double)right);
+			case TokenType.GreaterEqual:
+				checkNumberOperands(expr.Op, left, right);
+				return Variant.FromBool((double)left >= (double)right);
+			case TokenType.Less:
+				checkNumberOperands(expr.Op, left, right);
+				return Variant.FromBool((double)left < (double)right);
+			case TokenType.LessEqual:
+				checkNumberOperands(expr.Op, left, right);
+				return Variant.FromBool((double)left <= (double)right);
+
+			case TokenType.BangEqual:
+				return Variant.FromBool((left, right) switch
+				{
+					(String l, String r) => l != r,
+					(double l, double r) => l != r,
+					_ => throw new ArgumentException(),
+				});
+			case TokenType.DoubleEqual:
+				return Variant.FromBool((left, right) switch
+				{
+					(String l, String r) => l == r,
+					(double l, double r) => l == r,
+					_ => throw new ArgumentException(),
+				});
+		}
+
+		throw new ArgumentException($"bad binary op: {expr.Op}");
+	}
+
+	public object visitCallExpr(AST.Call expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitGroupingExpr(AST.Grouping expr)
+	{
+		return evaluate(expr.Expression);
+		throw new System.NotImplementedException();
+	}
+
+	public object visitIdentifierExpr(AST.Identifier expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitIfExpr(AST.If expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitIndexerExpr(AST.Indexer expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitLetExpr(AST.Let expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitListExpr(AST.List expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitLiteralExpr(AST.Literal expr)
+	{
+		return expr.Value;
+	}
+
+	public object visitRecordExpr(AST.Record expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitSelectorExpr(AST.Selector expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitSequenceExpr(AST.Sequence expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitUnaryExpr(AST.Unary expr)
+	{
+		object right = evaluate(expr.Right);
+		switch (expr.Op.type)
+		{
+			case TokenType.Minus:
+				checkNumberOperand(expr.Op, right);
+				return -(double)right;
+			case TokenType.Bang:
+				if (right is Variant v)
+				{
+					if (v == Variant.True)
+					{
+						return Variant.False;
+					}
+					else if (v == Variant.False)
+					{
+						return Variant.True;
+					}
+				}
+				throw new RuntimeError(expr.Op, "Boolean must be <true, false>");
+			default:
+				// Unreachable
+				throw new Exception($"bad unary op: {expr.Op}");
+		}
+	}
+
+	public object visitVariantExpr(AST.Variant expr)
+	{
+		throw new System.NotImplementedException();
+	}
+
+	public object visitWhenExpr(AST.When expr)
+	{
+		throw new System.NotImplementedException();
+	}
+}
\ No newline at end of file
diff --git a/Program.cs b/Program.cs
index da2f8e1..6ed5463 100644
--- a/Program.cs
+++ b/Program.cs
@@ -7,7 +7,9 @@ namespace Finn;
 
 class Program
 {
+	private static readonly Interpreter interpreter = new Interpreter();
 	static bool hadError = false;
+	static bool hadRuntimeError = true;
 
 	static void Main(string[] args)
 	{
@@ -30,6 +32,9 @@ class Program
 	{
 		var src = File.ReadAllText(path);
 		run(src);
+
+		if (hadError) Environment.Exit(65);
+		if (hadRuntimeError) Environment.Exit(70);
 	}
 
 	static void run(string src)
@@ -39,13 +44,13 @@ class Program
 		Parser parser = new Parser(tokens);
 		Expr? expression = parser.parse();
 
-		if (hadError || expression == null)
+		if (hadError)
 		{
 			hadError = false;
 			return;
 		}
 
-		Console.WriteLine(expression);
+		interpreter.Interpret(expression!);
 	}
 
 	static void runPrompt()
@@ -79,6 +84,12 @@ class Program
 		}
 	}
 
+	public static void runtimeError(Interpreter.RuntimeError err)
+	{
+		Console.Error.WriteLine($"{err.Message}\n[line {err.Token.line}]");
+		hadRuntimeError = true;
+	}
+
 	static void report(int line, String where, String message)
 	{
 		Console.WriteLine($"[line {line}] Error{where}: {message}");