diff --git a/jlx/src/main.cpp b/jlx/src/main.cpp
index 1a744a3..66a5812 100644
--- a/jlx/src/main.cpp
+++ b/jlx/src/main.cpp
@@ -63,5 +63,15 @@ int main(int argc, char** argv) {
 
 	std::cout << "Parsed " << rt->statements.size() << " statements" << std::endl;
 
+	auto type_checker = jlx::type_checker(rt->statements.begin(), rt->statements.end());
+	try {
+		type_checker.check();
+	} catch (jlx::type_error& err) {
+		std::cerr << "Type error: " << err.what() << std::endl;
+		return 1;
+	}
+
+	std::cout << "Type checking OK." << std::endl;
+
 	return 0;
 }
diff --git a/libjlx/modules/ast.cppm b/libjlx/modules/ast.cppm
index dbdae23..83b579c 100644
--- a/libjlx/modules/ast.cppm
+++ b/libjlx/modules/ast.cppm
@@ -17,8 +17,8 @@ namespace jlx {
 		Block,
 		FunctionDeclaration,
 		VariableDeclaration,
-		LiteralValue,
 		IfStatement,
+		ReturnStatement,
 	};
 
 	export enum expression_type {
@@ -52,7 +52,7 @@ namespace jlx {
 
 		}
 
-		explicit root_statement(std::vector<std::unique_ptr<statement>> statements, const token& t) : statement(Root, t), statements(std::move(statements)) {
+		root_statement(std::vector<std::unique_ptr<statement>> statements, const token& t) : statement(Root, t), statements(std::move(statements)) {
 
 		}
 
@@ -77,11 +77,25 @@ namespace jlx {
 		expression_type et = EtInvalid;
 		std::optional<std::string> evaluated_type;
 
-		virtual std::unique_ptr<expression> clone() const = 0;
+		[[nodiscard]] virtual std::unique_ptr<expression> clone() const = 0;
 
 		~expression() override = default;
 	};
 
+	export struct return_statement : public statement {
+		explicit return_statement(const token& t) : statement(ReturnStatement, t) {
+
+		}
+
+		return_statement(std::unique_ptr<expression> expression, const token& t) : statement(ReturnStatement, t), expression(std::move(expression)) {
+
+		}
+
+		std::unique_ptr<expression> expression;
+
+		~return_statement() override = default;
+	};
+
 	export struct block : public statement {
 		explicit block(const token& t) : statement(Block, t) {
 
@@ -455,6 +469,13 @@ namespace jlx {
 						}
 					}
 					fail_invalid_token(*current);
+					break;
+				case Keyword:
+					if (previous != nullptr) {
+						return previous;
+					}
+					fail_invalid_token(*current);
+					break;
 				default:
 					fail_invalid_token(*current);
 			}
@@ -531,6 +552,23 @@ namespace jlx {
 			return std::make_unique<function_declaration>(start, std::move(function_name), std::move(params), std::move(return_type), std::move(block));
 		}
 
+		std::unique_ptr<return_statement> parse_return_statement() {
+			auto start = *current;
+
+			if (current->type != Keyword || current->content != "return") {
+				fail_invalid_token(*current);
+			}
+
+			next(false);
+
+			std::unique_ptr<expression> expr = nullptr;
+			if (current != last) {
+				expr = parse_expression();
+			}
+
+			return std::make_unique<return_statement>(std::move(expr), start);
+		}
+
 		std::string parse_type(){
 			if (current->type != Identifier) {
 				fail_invalid_token(*current);
@@ -551,6 +589,8 @@ namespace jlx {
 					return parse_if_statement();
 				} else if (current->content == "fun" && top_level) {
 					return parse_function();
+				} else if (current->content == "return") {
+					return parse_return_statement();
 				}
 			}
 
diff --git a/libjlx/modules/tokenizer.cppm b/libjlx/modules/tokenizer.cppm
index 117c226..93fff78 100644
--- a/libjlx/modules/tokenizer.cppm
+++ b/libjlx/modules/tokenizer.cppm
@@ -80,13 +80,14 @@ namespace jlx {
 	export class tokenizer {
 		source_stream<char> source;
 
-		static constexpr std::array<std::string, 5> keywords = {{
+		static constexpr std::array<std::string, 6> keywords = {{
 			"if",
 			"else",
 			"fun",
 			//"struct",
 			"let",
-			"var"
+			"var",
+			"return"
 		}};
 
 		static constexpr std::array<char, 7> punctuations = {{
diff --git a/libjlx/modules/type_checker.cppm b/libjlx/modules/type_checker.cppm
index 0a0036b..09bd756 100644
--- a/libjlx/modules/type_checker.cppm
+++ b/libjlx/modules/type_checker.cppm
@@ -1,5 +1,6 @@
 module;
 
+#include <ranges>
 #include <stdexcept>
 #include <format>
 #include <string>
@@ -19,18 +20,20 @@ namespace jlx {
 
 	export class type_checker_runtime_context {
 	public:
-		virtual std::string get_identifier_type(const std::string_view&) const = 0;
-		virtual runtime_function get_function(const std::string_view&) const = 0;
+		virtual ~type_checker_runtime_context() = default;
+
+		virtual std::string_view get_identifier_type(const std::string_view&) const = 0;
+		virtual const runtime_function& get_function(const std::string_view&) const = 0;
 	};
 
-	export class type_error : std::runtime_error {
+	export class type_error : public std::runtime_error {
 	public:
 		explicit type_error(const token& t, const std::string_view& provided_type) : std::runtime_error(std::format("Type {} is invalid at {}:{}:{}", provided_type, t.source_file, t.line, t.col)) {
 
 		}
 	};
 
-	export class type_checker {
+	class type_checker_utils {
 		static std::size_t size_of(const std::string_view& type) {
 			if (type == "u8" || type == "i8" || type == "char") {
 				return 1;
@@ -57,7 +60,7 @@ namespace jlx {
 			return type == "f32" || type == "f64" || type == "comptime_float";
 		}
 
-
+	public:
 		static bool is_convertible_to(const std::string_view& source_type, const std::string_view& target_type) {
 			if (target_type == source_type) {
 				return true;
@@ -145,10 +148,10 @@ namespace jlx {
 			return false;
 		}
 
-		static void evaluate_expression_type(std::unique_ptr<expression>& expr, const type_checker_runtime_context& ctx) {
-			const auto& content = expr->t.content;
-			if (expr->et == EtLiteralValue) {
-				auto* literal_expression = dynamic_cast<literal_value*>(expr.get());
+		static void evaluate_expression_type(expression& expr, const type_checker_runtime_context& ctx) {
+			const auto& content = expr.t.content;
+			if (expr.et == EtLiteralValue) {
+				auto* literal_expression = dynamic_cast<literal_value*>(&expr);
 				if (literal_expression == nullptr) {
 					throw std::runtime_error("Internal type checker/AST error");
 				}
@@ -157,22 +160,25 @@ namespace jlx {
 				switch(literal_expression->t.type) {
 					case token_type::Number:
 						if (std::find(content.begin(), content.end(), '.') != content.end()) {
-							type = "comptime.float";
+							type = "comptime_float";
 						} else {
-							type = "comptime.int";
+							type = "comptime_int";
 						}
 						break;
 					case token_type::String:
-						type = "comptime.string";
+						type = "string";
+						break;
+					case token_type::Boolean:
+						type = "boolean";
 						break;
 					default:
 						throw std::runtime_error("Unsupported token for literal value expression");
 				}
 
-				expr->evaluated_type = type;
+				expr.evaluated_type = type;
 				return;
-			} else if (expr->et == EtSingleValueOperation) {
-				auto* single_op = dynamic_cast<single_operation*>(expr.get());
+			} else if (expr.et == EtSingleValueOperation) {
+				auto* single_op = dynamic_cast<single_operation*>(&expr);
 
 				if (single_op == nullptr) {
 					throw std::runtime_error("Internal type checker/AST error");
@@ -184,20 +190,20 @@ namespace jlx {
 
 				auto& opr = single_op->operand;
 				if (opr->evaluated_type == std::nullopt) {
-					evaluate_expression_type(opr, ctx);
+					evaluate_expression_type(*opr, ctx);
 				}
 
 				auto& op = single_op->operator_token.content;
 				auto base_type = opr->evaluated_type.has_value() ? opr->evaluated_type.value() : "void";
 				if (op == "+") {
 					if (type_is_integer(base_type) || type_is_float(base_type)) {
-						expr->evaluated_type = std::move(base_type);
+						expr.evaluated_type = std::move(base_type);
 						return;
 					}
 					throw type_error(opr->t, base_type);
 				} else if(op == "-") {
 					if (type_is_integer(base_type) || type_is_float(base_type)) {
-						expr->evaluated_type = std::move(base_type);
+						expr.evaluated_type = std::move(base_type);
 						//TODO: Maybe run immediately for 'comptime_int' and 'comptime_float'?
 						return;
 					}
@@ -207,13 +213,13 @@ namespace jlx {
 						throw type_error(opr->t, base_type);
 					}
 
-					expr->evaluated_type = "boolean";
+					expr.evaluated_type = "boolean";
 					return;
 				} else {
 					throw std::runtime_error("Cannot type-check expression: invalid operator for single-value operation");
 				}
-			} else if (expr->et == EtDualValueOperation) {
-				auto* dual_op = dynamic_cast<dual_operation*>(expr.get());
+			} else if (expr.et == EtDualValueOperation) {
+				auto* dual_op = dynamic_cast<dual_operation*>(&expr);
 
 				if (dual_op == nullptr) {
 					throw std::runtime_error("Internal type checker/AST error");
@@ -226,11 +232,11 @@ namespace jlx {
 				auto& expr0 = dual_op->first_operand;
 				auto& expr1 = dual_op->second_operand;
 				if (!expr0->evaluated_type.has_value()) {
-					evaluate_expression_type(expr0, ctx);
+					evaluate_expression_type(*expr0, ctx);
 				}
 
 				if (!expr1->evaluated_type.has_value()) {
-					evaluate_expression_type(expr1, ctx);
+					evaluate_expression_type(*expr1, ctx);
 				}
 
 				auto& op = dual_op->operator_token.content;
@@ -243,27 +249,27 @@ namespace jlx {
 					}
 
 					if (type0 == "string") {
-						expr->evaluated_type = "string";
+						expr.evaluated_type = "string";
 						return;
 					}
 
 					if (type0 == type1) {
-						expr->evaluated_type = std::move(type0);
+						expr.evaluated_type = std::move(type0);
 						return;
 					}
 
 					if ((type_is_integer(type0) && type_is_integer(type1)) || (type_is_float(type0) && type_is_float(type1))) {
-						expr->evaluated_type = size_of(type0) > size_of(type1) ? type0 : type1;
+						expr.evaluated_type = size_of(type0) > size_of(type1) ? type0 : type1;
 						return;
 					}
 
 					if (type_is_float(type0) && type1 == "comptime_int") {
-						expr->evaluated_type = type0;
+						expr.evaluated_type = type0;
 						return;
 					}
 
 					if (type_is_float(type1) && type0 == "comptime_int") {
-						expr->evaluated_type = type1;
+						expr.evaluated_type = type1;
 						return;
 					}
 
@@ -274,25 +280,25 @@ namespace jlx {
 					}
 
 					if ((type_is_integer(type0) && type_is_integer(type1)) || (type_is_float(type0) && type_is_float(type1))) {
-						expr->evaluated_type = size_of(type0) > size_of(type1) ? type0 : type1;
+						expr.evaluated_type = size_of(type0) > size_of(type1) ? type0 : type1;
 						return;
 					}
 
 
 					if (type_is_float(type0) && type1 == "comptime_int") {
-						expr->evaluated_type = type0;
+						expr.evaluated_type = type0;
 						return;
 					}
 
 					if (type_is_float(type1) && type0 == "comptime_int") {
-						expr->evaluated_type = type1;
+						expr.evaluated_type = type1;
 						return;
 					}
 
 					throw std::runtime_error("Unsupported type combination for operator '+'");
 				} else if (op == "==" || op == "!=") {
 					if (is_convertible_to(type1, type0) || is_convertible_to(type0, type1)) {
-						expr->evaluated_type = "boolean";
+						expr.evaluated_type = "boolean";
 						return;
 					}
 
@@ -310,11 +316,11 @@ namespace jlx {
 						throw type_error(expr1->t, type1);
 					}
 
-					expr->evaluated_type = "boolean";
+					expr.evaluated_type = "boolean";
 					return;
 				}
-			} else if (expr->et == EtFunctionCall) {
-				auto* call = dynamic_cast<function_call*>(expr.get());
+			} else if (expr.et == EtFunctionCall) {
+				auto* call = dynamic_cast<function_call*>(&expr);
 
 				if (call == nullptr) {
 					throw std::runtime_error("Internal type checker/AST error");
@@ -332,7 +338,7 @@ namespace jlx {
 					auto& arg = call->arguments[i];
 
 					if (!arg->evaluated_type.has_value()) {
-						evaluate_expression_type(arg, ctx);
+						evaluate_expression_type(*arg, ctx);
 					}
 
 					auto src = arg->evaluated_type.has_value() ? arg->evaluated_type.value() : "void";
@@ -341,20 +347,222 @@ namespace jlx {
 					}
 				}
 
-				expr->evaluated_type = fn.return_type;
+				expr.evaluated_type = fn.return_type;
 				return;
-			} else if (expr->et == EtIdentifier) {
-				auto* ident_expr = dynamic_cast<identifier_expression*>(expr.get());
+			} else if (expr.et == EtIdentifier) {
+				auto* ident_expr = dynamic_cast<identifier_expression*>(&expr);
 
 				if (ident_expr == nullptr) {
 					throw std::runtime_error("Internal type checker/AST error");
 				}
 
-				expr->evaluated_type = ctx.get_identifier_type(ident_expr->name);
+				expr.evaluated_type = ctx.get_identifier_type(ident_expr->name);
 				return;
 			}
 
 			throw std::runtime_error("Cannot type-check expression: Unknown expression");
 		}
 	};
+	export template<class T>
+	concept statement_iterator = requires() {
+		requires std::same_as<typename T::value_type, std::unique_ptr<statement>>;
+		std::bidirectional_iterator<T>;
+	};
+
+	struct type_checker_scope {
+		std::unordered_map<std::string, std::string> variables;
+		std::unordered_map<std::string, runtime_function> functions;
+		std::optional<std::string> intended_return_type;
+	};
+
+	export template<statement_iterator T, std::sentinel_for<T> E>
+	class type_checker : public type_checker_runtime_context {
+	private:
+		T current;
+		E end;
+		std::vector<type_checker_scope> scopes;
+
+		void check_statement(statement& s, bool skip_block_scope = false) {
+			switch (s.type) {
+				case Root: {
+						auto* root = dynamic_cast<root_statement*>(&s);
+
+						for (auto& st : root->statements) {
+							check_statement(*st);
+						}
+					}
+					break;
+				case Block: {
+						auto* blk = dynamic_cast<block*>(&s);
+
+						if (!skip_block_scope) {
+							scopes.emplace_back(type_checker_scope{});
+						}
+
+						for (auto& st : blk->statements) {
+							check_statement(*st);
+						}
+
+						if (!skip_block_scope) {
+							scopes.pop_back();
+						}
+					}
+					break;
+				case IfStatement: {
+						auto* if_stmt = dynamic_cast<if_statement*>(&s);
+
+						auto& cond = if_stmt->condition;
+						if (!cond->evaluated_type.has_value()) {
+							type_checker_utils::evaluate_expression_type(*if_stmt->condition, *this);
+						}
+
+						auto tp = cond->evaluated_type.has_value() ? cond->evaluated_type.value() : "void";
+
+						if (tp != "boolean") {
+							throw type_error(cond->t, tp);
+						}
+
+						check_statement(*if_stmt->block);
+					}
+					break;
+				case VariableDeclaration: {
+						auto* decl = dynamic_cast<variable_declaration*>(&s);
+
+						if (has_variable(decl->name)) {
+							throw std::runtime_error("Duplicate variable name detected");
+						}
+
+						std::optional<std::string> init_type = std::nullopt;
+						if (decl->initial_expression != nullptr) {
+							if (!decl->initial_expression->evaluated_type.has_value()) {
+								type_checker_utils::evaluate_expression_type(*decl->initial_expression, *this);
+							}
+							init_type = decl->initial_expression->evaluated_type;
+						}
+
+						auto type = decl->type;
+
+						if (init_type.has_value()) {
+							if (!type.has_value()) {
+								type = init_type;
+							} else if (!type_checker_utils::is_convertible_to(init_type.value(), type.value())) {
+								throw type_error(decl->initial_expression->t, init_type.value());
+							}
+						}
+
+						if (!type.has_value()) {
+							throw std::runtime_error("Cannot infer variable declaration type");
+						}
+
+						scopes[scopes.size() - 1].variables.insert_or_assign(decl->name, type.value());
+					}
+					break;
+				case FunctionDeclaration: {
+						auto* decl = dynamic_cast<function_declaration*>(&s);
+
+						if (has_function(decl->name)) {
+							throw std::runtime_error("Duplicate function name detected");
+						}
+
+						if (!decl->return_type.has_value()) {
+							decl->return_type = "void";
+						}
+
+						runtime_function f;
+						f.return_type = decl->return_type.value();
+
+						for (auto& a : decl->parameters) {
+							f.arguments.emplace_back(a.type);
+						}
+
+						scopes[scopes.size() - 1].functions.insert_or_assign(decl->name, f);
+
+						auto& function_scope = scopes.emplace_back(type_checker_scope{});
+						function_scope.intended_return_type = decl->return_type;
+
+						check_statement(*decl->body, true);
+
+						scopes.pop_back();
+					}
+					break;
+				case Expression: {
+						auto* expr = dynamic_cast<expression*>(&s);
+
+						type_checker_utils::evaluate_expression_type(*expr, *this);
+					}
+					break;
+				case ReturnStatement: {
+						auto* rtr = dynamic_cast<return_statement*>(&s);
+
+						auto& scope = scopes.back();
+
+						if (rtr->expression != nullptr && !rtr->expression->evaluated_type.has_value()) {
+							type_checker_utils::evaluate_expression_type(*rtr->expression, *this);
+						}
+
+						auto rvl = scope.intended_return_type.has_value() ? scope.intended_return_type.value() : "void";
+						auto exvl = rtr->expression != nullptr && rtr->expression->evaluated_type.has_value() ? rtr->expression->evaluated_type.value() : "void";
+
+						if (!type_checker_utils::is_convertible_to(exvl, rvl)) {
+							throw type_error(rtr->expression->t, exvl);
+						}
+					}
+					break;
+			}
+		}
+
+		[[nodiscard]] bool has_variable(const std::string_view& name) const {
+			auto& scope = scopes[scopes.size() - 1];
+			auto itx = scope.variables.find(std::string(name));
+			if (itx != scope.variables.end()) {
+					return true;
+			}
+
+			return false;
+		}
+
+		[[nodiscard]] bool has_function(const std::string_view& name) const {
+			for (const auto& scope : std::ranges::reverse_view(scopes)) {
+				auto itx = scope.functions.find(std::string(name));
+				if (itx != scope.functions.end()) {
+					return true;
+				}
+			}
+
+			return false;
+		}
+	public:
+		type_checker(T start, E end) : current(start), end(end) {
+			scopes.emplace_back();
+		}
+
+		void check() {
+			while (current != end) {
+				check_statement(**current);
+
+				++current;
+			}
+		}
+
+		[[nodiscard]] std::string_view get_identifier_type(const std::string_view &name) const override {
+			for (const auto & scope : std::ranges::reverse_view(scopes)) {
+				auto itx = scope.variables.find(std::string(name));
+				if (itx != scope.variables.end()) {
+					return itx->second;
+				}
+			}
+
+			throw std::runtime_error("Missing identifier");
+		}
+
+		[[nodiscard]] const runtime_function& get_function(const std::string_view &name) const override {
+			for (const auto& scope : std::ranges::reverse_view(scopes)) {
+				if (auto itx = scope.functions.find(std::string(name)); itx != scope.functions.end()) {
+					return itx->second;
+				}
+			}
+
+			throw std::runtime_error("Missing function");
+		}
+	};
 }
\ No newline at end of file