diff --git a/src/common/util.d b/src/common/util.d
index 14e0917..f8dc60d 100644
--- a/src/common/util.d
+++ b/src/common/util.d
@@ -6,3 +6,4 @@ template defaultCtor(){
 			this.tupleof[i] = a;
 	}
 }
+
diff --git a/src/jlox/environment.d b/src/jlox/environment.d
index 06023dc..baee5fe 100644
--- a/src/jlox/environment.d
+++ b/src/jlox/environment.d
@@ -20,6 +20,15 @@ class Environment{
 			return enclosing.get(name);
 		throw new RuntimeError(name, "Undefined variable '" ~ name.lexeme ~ "'.");
 	}
+	Environment ancestor(uint distance){
+		Environment environment = this;
+		for(uint i = 0; i < distance; i++)
+			environment = environment.enclosing;
+		return environment;
+	}
+	TValue getAt(uint distance, string name){
+		return ancestor(distance).values[name];
+	}
 	void assign(Token name, TValue value){
 		if(name.lexeme in values)
 			return values[name.lexeme] = value;
@@ -27,5 +36,8 @@ class Environment{
 			return enclosing.assign(name, value);
 		throw new RuntimeError(name, "Undefined variable '" ~ name.lexeme ~ "'.");
 	}
+	void assignAt(uint distance, Token name, TValue value){
+		ancestor(distance).values[name.lexeme] = value;
+	}
 }
 
diff --git a/src/jlox/expr.d b/src/jlox/expr.d
index 2aea4b1..ed313f6 100644
--- a/src/jlox/expr.d
+++ b/src/jlox/expr.d
@@ -21,7 +21,7 @@ abstract class Expr{
 		R visit(Unary expr);
 		R visit(Variable expr);
 	}
-	private alias rTypes = AliasSeq!(string, TValue);
+	private alias rTypes = AliasSeq!(string, TValue, void);
 	static foreach(T; rTypes)
 		abstract T accept(Visitor!T visitor);
 	private template defCtorAndAccept(){
diff --git a/src/jlox/interpreter.d b/src/jlox/interpreter.d
index 157d0a5..9af9dd3 100644
--- a/src/jlox/interpreter.d
+++ b/src/jlox/interpreter.d
@@ -97,7 +97,10 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 			return;
 		throw new RuntimeError(operator, "Operand must be a number.");
 	}
-
+	private uint[Expr] locals;
+	void resolve(Expr expr, uint depth){
+		locals[expr] = depth;
+	}
 
 	void visit(Stmt.Block stmt){
 		executeBlock(stmt.statements, new Environment(environment));
@@ -212,11 +215,20 @@ class Interpreter : Stmt.Visitor!void, Expr.Visitor!TValue {
 		return func.call(this, arguments.array);
 	}
 	TValue visit(Expr.Variable expr){
-		return environment.get(expr.name);
+		return lookUpVariable(expr.name, expr);
+	}
+	private TValue lookUpVariable(Token name, Expr expr){
+		if(const distance = expr in locals)
+			return environment.getAt(*distance, name.lexeme);
+		else
+			return globals.get(name);
 	}
 	TValue visit(Expr.Assign expr){
 		TValue value = evaluate(expr.value);
-		environment.assign(expr.name, value);
+		if(const distance = expr in locals)
+			environment.assignAt(*distance, expr.name, value);
+		else
+			globals.assign(expr.name, value);
 		return value;
 	}
 }
diff --git a/src/jlox/main.d b/src/jlox/main.d
index 51e0e33..71a7541 100644
--- a/src/jlox/main.d
+++ b/src/jlox/main.d
@@ -14,6 +14,11 @@ import jlox.parser;
 import jlox.interpreter;
 import jlox.expr;
 import jlox.stmt;
+import jlox.resolver;
+
+enum RetVal{
+	success = 0, other = 1, runtime = 2
+}
 
 private class MainException : Exception{
 	this(){
@@ -52,7 +57,11 @@ static class Lox{
 
 		Parser parser = new Parser(tokens);
 		Stmt[] statements = parser.parse();
+		if(hadError)
+			return;
 
+		Resolver resolver = new Resolver(interpreter);
+		resolver.resolve(statements);
 		if(hadError)
 			return;
 
@@ -85,7 +94,7 @@ int main(string[] argv){
 	try{
 		if(args.arg("path") && args.option("command")){
 			stderr.writeln("Cant have both path and --cmd");
-			return 3;
+			return RetVal.other;
 		}
 		if(auto cmd = args.option("command"))
 			Lox.run(cmd);
@@ -94,8 +103,8 @@ int main(string[] argv){
 		else
 			Lox.runPrompt();
 	} catch(MainException rte){
-		return Lox.hadError ? 1 : 2;
+		return Lox.hadError ? RetVal.other : RetVal.runtime;
 	}
-	return 0;
+	return RetVal.success;
 }
 
diff --git a/src/jlox/resolver.d b/src/jlox/resolver.d
new file mode 100644
index 0000000..0b02a35
--- /dev/null
+++ b/src/jlox/resolver.d
@@ -0,0 +1,143 @@
+module jlox.resolver;
+
+import std.container.slist;
+import std.stdio;
+import std.range : enumerate;
+
+import jlox.main;
+import jlox.token;
+import jlox.stmt;
+import jlox.expr;
+import jlox.interpreter;
+import common.util;
+
+private enum FunctionType { NONE, FUNCTION }
+
+class Resolver : Stmt.Visitor!void, Expr.Visitor!void {
+	private Interpreter interpreter;
+	this(Interpreter interpreter){
+		this.interpreter = interpreter;
+	}
+	private FunctionType currentFunction = FunctionType.NONE;
+	private SList!(bool[string]) scopes;
+	private void resolve(Stmt stmt) => stmt.accept(this);
+	private void resolve(Expr expr) => expr.accept(this);
+	void resolve(Stmt[] statements){
+		foreach(statement; statements)
+			resolve(statement);
+	}
+	private void resolveLocal(Expr expr, Token name){
+		foreach(i, scp; scopes[].enumerate){
+			if(name.lexeme in scp){
+				interpreter.resolve(expr, cast(uint)i);
+				return;
+			}
+		}
+	}
+	private void resolveFunction(Stmt.Function func, FunctionType type){
+		FunctionType enclosingFunction = currentFunction;
+		currentFunction = type;
+		scope(exit)
+			currentFunction = enclosingFunction;
+		beginScope();
+		foreach(param; func.params){
+			declare(param);
+			define(param);
+		}
+		resolve(func.body);
+		endScope();
+	}
+	private void endScope() => scopes.removeFront();
+	private void beginScope(){
+		bool[string] s;
+		scopes.insertFront(s);
+	}
+	private void declare(Token name){
+		if(scopes.empty)
+			return;
+		bool[string] scp = scopes.front;
+		if(name.lexeme in scp)
+			Lox.error(name, "Already a variable with this name in this scope.");
+		scp[name.lexeme] = false;
+	}
+	private void define(Token name){
+		if(scopes.empty)
+			return;
+		scopes.front[name.lexeme] = true;
+	}
+
+	void visit(Stmt.Block stmt){
+		beginScope();
+		resolve(stmt.statements);
+		endScope();
+	}
+	void visit(Stmt.Var stmt){
+		declare(stmt.name);
+		if(stmt.initialiser !is null)
+			resolve(stmt.initialiser);
+		define(stmt.name);
+	}
+	void visit(Stmt.Function stmt){
+		declare(stmt.name);
+		define(stmt.name);
+		resolveFunction(stmt, FunctionType.FUNCTION);
+	}
+	void visit(Expr.Variable expr){
+		if(!scopes.empty && scopes.front.get(expr.name.lexeme, true) == false)
+			Lox.error(expr.name, "Can't read local variable in its own initializer.");
+		resolveLocal(expr, expr.name);
+	}
+	void visit(Expr.Assign expr){
+		resolve(expr.value);
+		resolveLocal(expr, expr.name);
+	}
+
+	void visit(Stmt.Expression stmt){
+		resolve(stmt.expression);
+	}
+	void visit(Stmt.If stmt){
+		resolve(stmt.condition);
+		resolve(stmt.thenBranch);
+		if(stmt.elseBranch !is null)
+			resolve(stmt.elseBranch);
+	}
+	void visit(Stmt.Print stmt){
+		version(LoxPrintMultiple){
+			foreach(expr; stmt.expressions)
+				resolve(expr);
+		} else {
+			resolve(stmt.expression);
+		}
+	}
+	void visit(Stmt.Return stmt){
+		if(currentFunction == FunctionType.NONE)
+			Lox.error(stmt.keyword, "Can't return from top-level code.");
+		if(stmt.value !is null)
+			resolve(stmt.value);
+	}
+	void visit(Stmt.While stmt){
+		resolve(stmt.condition);
+		resolve(stmt.body);
+	}
+	void visit(Expr.Binary expr){
+		resolve(expr.left);
+		resolve(expr.right);
+	}
+	void visit(Expr.Call expr){
+		resolve(expr.callee);
+		foreach(argument; expr.arguments)
+			resolve(argument);
+	}
+	void visit(Expr.Grouping expr){
+		resolve(expr.expression);
+	}
+	void visit(Expr.Literal expr){}
+	void visit(Expr.Logical expr){
+		resolve(expr.left);
+		resolve(expr.right);
+	}
+	void visit(Expr.Unary expr){
+		resolve(expr.right);
+	}
+}
+
diff --git a/test/all.d b/test/all.d
index 71de62f..2145c73 100755
--- a/test/all.d
+++ b/test/all.d
@@ -1,23 +1,49 @@
 #!/bin/env rdmd
 
 import std.process;
+import std.concurrency;
 import std.conv;
+import std.string, std.format;
+import std.algorithm, std.range;
 
 void main(){
-	string fib(uint n){
-		string r = "";
-		double a = 0;
-		double temp;
-		for(double b = 1; a <= n; b = temp + b){
-			r ~= a.to!string ~ "\n";
-			temp = a;
-			a = b;
-		}
-		return r;
-	}
-	assert([ "./lox", "test/closure.lox" ].execute.output == "1\n2\n");
-	assert([ "./lox", "test/fib_for.lox" ].execute.output == fib(6765));
-	assert([ "./lox", "test/fib_recursive.lox" ].execute.output == fib(34));
-	assert([ "./lox", "test/fib_closure.lox" ].execute.output == fib(34));
+	auto scheduler = new ThreadScheduler();
+	scheduler.spawn((){ "./test/closure.lox".run.output.match("1\n2\n"); });
+	scheduler.spawn((){ "./test/scope.lox".run.output.match("global first first second first ".replace(' ', '\n').repeat(2).join("\n")); });
+	scheduler.spawn((){ "./test/fib_for.lox".run.output.match(fib(6765)); });
+	scheduler.spawn((){ "./test/fib_recursive.lox".run.output.match(fib(34)); });
+	scheduler.spawn((){ "./test/fib_closure.lox".run.output.match(fib(34)); });
+
+	scheduler.spawn((){ "./test/err/already_defined.lox".shouldFail(RetVal.other, "Already a variable with this name"); });
+	scheduler.spawn((){ "./test/err/undefined_var.lox".shouldFail(RetVal.runtime, "Undefined variable"); });
+	scheduler.spawn((){ "./test/err/self_ref_vardecl.lox".shouldFail(RetVal.runtime, "Undefined variable"); });
+	scheduler.spawn((){ "./test/err/invalid_syntax.lox".shouldFail(RetVal.other); });
+	scheduler.spawn((){ "./test/err/global_scope_return.lox".shouldFail(RetVal.other, "Can't return from top-level code"); });
+}
+
+enum RetVal{
+	success = 0, other = 1, runtime = 2
+}
+
+string fib(uint n){
+	string r = "";
+	double a = 0;
+	double temp;
+	for(double b = 1; a <= n; b = temp + b){
+		r ~= a.to!string ~ "\n";
+		temp = a;
+		a = b;
+	}
+	return r;
+}
+auto run(string file) => [ "./lox", file ].execute;
+void match(string res, string correct){
+	assert(res == correct, "Match failed\n-- Got --\n%s\n-- Expected --\n%s".format(res, correct));
+}
+void shouldFail(string file, int code = 1, string msg = null){
+	auto c = file.run;
+	assert(c.status == code, "Expected %s to fail with code %d but got %d".format(file, code, c.status));
+	assert(!msg || c.output.toLower.indexOf(msg.toLower) >= 0, "ShouldFail %s failed\n-- Got --\n%s\n-- Expected --\n%s".format(file, c.output, msg));
+	assert(c.output.indexOf("_Dmain") == -1, "ShouldFail %s got D exception\n%s".format(file, c.output));
 }
 
diff --git a/test/err/already_defined.lox b/test/err/already_defined.lox
new file mode 100644
index 0000000..6d142d4
--- /dev/null
+++ b/test/err/already_defined.lox
@@ -0,0 +1,6 @@
+
+fun bad(){
+	var a = "first";
+	var a = "second";
+}
+
diff --git a/test/err/global_scope_return.lox b/test/err/global_scope_return.lox
new file mode 100644
index 0000000..c0658c2
--- /dev/null
+++ b/test/err/global_scope_return.lox
@@ -0,0 +1,3 @@
+
+return ":)";
+
diff --git a/test/err/invalid_syntax.lox b/test/err/invalid_syntax.lox
new file mode 100644
index 0000000..24823b1
--- /dev/null
+++ b/test/err/invalid_syntax.lox
@@ -0,0 +1,3 @@
+
+foo
+
diff --git a/test/err/self_ref_vardecl.lox b/test/err/self_ref_vardecl.lox
new file mode 100644
index 0000000..e58a127
--- /dev/null
+++ b/test/err/self_ref_vardecl.lox
@@ -0,0 +1,3 @@
+
+var x = x + 1;
+
diff --git a/test/err/undefined_var.lox b/test/err/undefined_var.lox
new file mode 100644
index 0000000..b3162ca
--- /dev/null
+++ b/test/err/undefined_var.lox
@@ -0,0 +1,3 @@
+
+print hello;
+
diff --git a/test/scope.lox b/test/scope.lox
new file mode 100644
index 0000000..4b9eadc
--- /dev/null
+++ b/test/scope.lox
@@ -0,0 +1,19 @@
+
+var a = "global";
+
+fun f(){
+	print a;
+	var a = "first";
+	print a;
+	{
+		print a;
+		var a = "second";
+		print a;
+	}
+	print a;
+}
+
+f();
+print "";
+f();
+
diff --git a/test/test.lox b/test/test.lox
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/test/test.lox
@@ -0,0 +1 @@
+