diff --git a/CMakeLists.txt b/CMakeLists.txt
index a5576f1..5e8713f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,5 +2,5 @@ cmake_minimum_required(VERSION 3.0)
 project(lily)
 
 set(CMAKE_CXX_STANDARD 14)
-add_executable(lily src/main.cpp src/parser.cpp src/parser.c src/type.cpp src/type_manager.cpp)
+add_executable(lily src/main.cpp src/parser.cpp src/parser.c src/type.cpp src/type_manager.cpp src/ast.cpp src/type_checker.cpp)
 target_include_directories(lily PUBLIC src)
diff --git a/src/ast.cpp b/src/ast.cpp
new file mode 100644
index 0000000..403ddf7
--- /dev/null
+++ b/src/ast.cpp
@@ -0,0 +1,73 @@
+#include "ast.hpp"
+#include "type_checker.hpp"
+#include "error.hpp"
+
+namespace lily {
+    type* ast_num::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        std::cout << "checking number" << std::endl;
+        return mgr.require_type("Int");
+    }
+
+    type* ast_var::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        std::cout << "checking variable" << std::endl;
+        return env->get_identifier_type(name);
+    }
+
+    type* ast_app::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        std::cout << "checking application" << std::endl;
+        type* ltype = left->check(mgr, env);
+        type* rtype = right->check(mgr, env);
+
+        // We LHS has to be a function, so unify LHS with that.
+        type_func* f = mgr.create_type<type_func>(nullptr, nullptr);
+        // Input type of function is known - it's rtype.
+        f->left = rtype;
+        // Output is not known - create parameter for it.
+        type* new_param = mgr.create_type<type_parameter>();
+        f->right = new_param;
+
+        // Unify ltype with the function (we know it has to be one!)
+        if(!ltype->unify_with(f))
+            throw error("unable to unify application type");
+        
+        return new_param;
+    }
+
+    type* ast_op::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        type* ltype = left->check(mgr, env);
+        type* rtype = right->check(mgr, env);
+
+        // We know the thing has to be a nunmber, so we unify with number type.
+        type* inttype = mgr.require_type("Int");
+        if(!ltype->unify_with(inttype))
+            throw error("left hand side of operator must be a number.");
+        if(!rtype->unify_with(inttype))
+            throw error("right hand side of operator must be a number.");
+
+        // We return an integer.
+        return inttype;
+    }
+
+    type* ast_let::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        if(env->identifier_exists(name))
+            throw error("invalid redefinition of variable.");
+        type* etype = expr->check(mgr, env);
+        auto new_env = env->with_type(name, etype);
+        return in->check(mgr, new_env);
+    }
+
+    type* ast_letrec::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        if(env->identifier_exists(name))
+            throw error("invalid redefinition of variable.");
+        type* variable_type = mgr.create_type<type_parameter>();
+        auto new_env = env->with_type(name, variable_type);
+        type* etype = expr->check(mgr, new_env);
+        if(!variable_type->unify_with(etype))
+            throw error("incompatible type for variable");
+        return in->check(mgr, new_env);
+    }
+
+    type* ast_case::check(type_manager& mgr, std::shared_ptr<type_env> env) {
+        throw error("unimplemented");
+    }
+}
diff --git a/src/ast.hpp b/src/ast.hpp
index 0086c58..0604aac 100644
--- a/src/ast.hpp
+++ b/src/ast.hpp
@@ -2,10 +2,15 @@
 #include <string>
 #include <memory>
 #include "pattern.hpp"
+#include "type.hpp"
+#include "type_manager.hpp"
 
 namespace lily {
+    class type_env;
+
     struct ast {
         virtual ~ast() = default;
+        virtual type* check(type_manager& mgr, std::shared_ptr<type_env> env) = 0;
     };
 
     typedef std::unique_ptr<ast> ast_ptr;
@@ -15,6 +20,7 @@ namespace lily {
 
         ast_num(int i) : num(i) {}
         ~ast_num() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct ast_var : ast {
@@ -23,6 +29,7 @@ namespace lily {
         ast_var(std::string n) :
             name(std::move(n)) {}
         ~ast_var() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct ast_app : ast {
@@ -32,6 +39,7 @@ namespace lily {
         ast_app(ast_ptr l, ast_ptr r) :
             left(std::move(l)), right(std::move(r)) {}
         ~ast_app() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct ast_op : ast {
@@ -47,6 +55,7 @@ namespace lily {
         ast_op(enum op o, ast_ptr l, ast_ptr r) :
             op(o), left(std::move(l)), right(std::move(r)) {}
         ~ast_op() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct ast_let : ast {
@@ -57,12 +66,14 @@ namespace lily {
         ast_let(std::string n, ast_ptr e, ast_ptr i) :
             name(std::move(n)), expr(std::move(e)), in(std::move(i)) {}
         ~ast_let() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct ast_letrec : ast_let {
         ast_letrec(std::string n, ast_ptr e, ast_ptr i) :
             ast_let(std::move(n), std::move(e), std::move(i)) {}
         ~ast_letrec() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 
     struct ast_case : ast {
@@ -75,5 +86,6 @@ namespace lily {
         std::vector<branch> branches;
 
         ~ast_case() = default;
+        type* check(type_manager& mgr, std::shared_ptr<type_env> env);
     };
 }
diff --git a/src/main.cpp b/src/main.cpp
index 7afe916..f2a6908 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -8,7 +8,7 @@ int main() {
                 "data Bool = { True, False }\n"
                 "data Color = { Red, Black }\n"
                 "data IntList = { Nil, Cons(Int, IntList) }\n"
-                "defn add x y = { x + y }");
+                "defn add x y = { x + add x x }");
     } catch(lily::error& e) {
         std::cout << e.message << std::endl;
     }
diff --git a/src/parser.cpp b/src/parser.cpp
index c34d4e6..0116f04 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -3,6 +3,7 @@ extern "C" {
 }
 #include "parser.hpp"
 #include "pattern.hpp"
+#include "type_checker.hpp"
 #include <memory>
 
 namespace lily {
@@ -238,7 +239,47 @@ namespace lily {
         }
         
         program_ptr prog = build_program(into, s.c_str());
+        prog->check();
         pgs_free_tree(into);
         return prog;
     }
+
+    void program::check() {
+        // Each function has an environment, which will be used
+        // as base for type checking.
+        std::map<std::string, std::shared_ptr<type_env>> function_envs;
+        auto base_env = std::make_shared<type_env>();
+
+        // First step is to collect all function types.
+        for(auto& pair : functions) {
+            // Create a local function environment.
+            auto function_env = std::make_shared<type_env>();
+            function_env->set_parent(base_env.get());
+            function_envs[pair.first] = function_env;
+
+            // We'll be building up the function type.
+            // Create the return type parameter
+            type* return_type = type_mgr.create_type<type_parameter>();
+            type* current_type = return_type;
+
+            // Create type parameters for every variable
+            // We also want to place them in a local environment
+            for(int i = 0; i < pair.second.params.size(); i++) {
+                const std::string& str =
+                    pair.second.params[pair.second.params.size() - i - 1];
+                type* variable_param = type_mgr.create_type<type_parameter>();
+                function_env->set_type(str, variable_param);
+                current_type =
+                    type_mgr.create_type<type_func>(variable_param, current_type);
+            }
+
+            // Store function in env.
+            base_env->set_type(pair.first, current_type);
+        }
+
+        // Now that we have collected the functions, check their bodies.
+        for(auto& pair : functions) {
+            pair.second.body->check(type_mgr, function_envs[pair.first]);
+        }
+    }
 }
diff --git a/src/parser.hpp b/src/parser.hpp
index efbc2ea..23cd90a 100644
--- a/src/parser.hpp
+++ b/src/parser.hpp
@@ -11,6 +11,8 @@ namespace lily {
     struct program {
         type_manager type_mgr;
         std::map<std::string, function> functions;
+
+        void check();
     };
 
     typedef std::unique_ptr<program> program_ptr;
diff --git a/src/type.cpp b/src/type.cpp
index 0925c25..d1aea8f 100644
--- a/src/type.cpp
+++ b/src/type.cpp
@@ -1,4 +1,5 @@
 #include "type.hpp"
+#include "error.hpp"
 
 namespace lily {
     type_data::constructor* type_data::create_constructor(const std::string& name,
@@ -12,4 +13,49 @@ namespace lily {
         constructors.push_back(std::move(new_constructor));
         return raw_ptr;
     }
+
+    bool try_unify_pram(type* substitute_with, type* substitute_in) {
+        type_parameter* param = dynamic_cast<type_parameter*>(substitute_in);
+        if(!param) return false;
+        if(!param->actual_type) {
+            param->actual_type = substitute_with;
+            return true;
+        } else {
+            return substitute_with->unify_with(param->actual_type);
+        }
+    }
+
+    bool type_internal::unify_with(type* other) {
+        type_internal* other_int = dynamic_cast<type_internal*>(other);
+        if(other_int && other_int->type_id == type_id) return true;
+        if(try_unify_pram(this, other)) return true;
+        return false;
+    }
+
+    bool type_parameter::unify_with(type* other) {
+        if(this == other) return true;
+        if(!actual_type) {
+            actual_type = other;
+            return true;
+        }
+        if(try_unify_pram(this, other)) return true;
+        if(actual_type->unify_with(other)) return true;
+        return false;
+    }
+
+    bool type_data::unify_with(type* other) {
+        type_data* other_data = dynamic_cast<type_data*>(other);
+        if(other_data && other_data->type_id == type_id) return true;
+        if(try_unify_pram(this, other)) return true;
+        return false;
+    }
+
+    bool type_func::unify_with(type* other) {
+        type_func* other_func = dynamic_cast<type_func*>(other);
+        if(try_unify_pram(this, other)) return true;
+        if(other_func &&
+                left->unify_with(other_func->left) &&
+                right->unify_with(other_func->right)) return true;
+        return false;
+    }
 }
diff --git a/src/type.hpp b/src/type.hpp
index acef15a..66a0133 100644
--- a/src/type.hpp
+++ b/src/type.hpp
@@ -11,12 +11,19 @@ namespace lily {
 
     struct type {
         virtual ~type() = default;
+        virtual bool unify_with(type* other) = 0;
     };
 
     struct type_internal : type {
         int type_id;
 
         type_internal(int id) : type_id(id) {}
+        bool unify_with(type* other);
+    };
+
+    struct type_parameter : type {
+        type* actual_type = nullptr;
+        bool unify_with(type* other);
     };
 
     struct type_data : type {
@@ -31,6 +38,7 @@ namespace lily {
 
         type_data(int id) : type_id(id) {}
         constructor* create_constructor(const std::string& name, std::vector<type*>&& params);
+        bool unify_with(type* other);
     };
 
     struct type_func : type {
@@ -39,6 +47,7 @@ namespace lily {
 
         type_func(type* l, type* r) :
             left(l), right(r) {}
+        bool unify_with(type* other);
     };
 }
 
diff --git a/src/type_checker.cpp b/src/type_checker.cpp
new file mode 100644
index 0000000..b73fc1e
--- /dev/null
+++ b/src/type_checker.cpp
@@ -0,0 +1,32 @@
+#include "type_checker.hpp"
+#include "error.hpp"
+
+namespace lily {
+    bool type_env::identifier_exists(const std::string& name) {
+        return identifier_types.count(name) != 0 || (parent && parent->identifier_exists(name));
+    }
+
+    type* type_env::get_identifier_type(const std::string& name) {
+        std::cout << "looking up " << name << std::endl;
+        if(!identifier_types.count(name)) {
+            if(parent) return parent->get_identifier_type(name);
+            throw error("unknown variable");
+        }
+        return identifier_types[name];
+    }
+
+    std::shared_ptr<type_env> type_env::with_type(const std::string& name, type* ntype) {
+        auto child_env = std::make_shared<type_env>();
+        child_env->parent = this;
+        child_env->identifier_types[name] = ntype;
+        return child_env;
+    }
+
+    void type_env::set_type(const std::string& name, type* ntype) {
+        identifier_types[name] = ntype;
+    }
+
+    void type_env::set_parent(type_env* parent) {
+        this->parent = parent;
+    }
+}
diff --git a/src/type_checker.hpp b/src/type_checker.hpp
new file mode 100644
index 0000000..e909b67
--- /dev/null
+++ b/src/type_checker.hpp
@@ -0,0 +1,22 @@
+#pragma once
+#include <map>
+#include "type.hpp"
+#include "ast.hpp"
+
+namespace lily {
+    class type_env {
+        private:
+            type_env* parent = nullptr;
+            std::map<std::string, type*> identifier_types;
+
+        public:
+            bool identifier_exists(const std::string& name);
+            type* get_identifier_type(const std::string& name);
+            void unify(type* l, type* r);
+
+            std::shared_ptr<type_env> with_type(const std::string& name, type* ntype);
+            void set_type(const std::string& name, type* ntype);
+
+            void set_parent(type_env* new_parent);
+    };
+}
diff --git a/src/type_manager.cpp b/src/type_manager.cpp
index e495f8e..92b9c58 100644
--- a/src/type_manager.cpp
+++ b/src/type_manager.cpp
@@ -34,8 +34,8 @@ namespace lily {
         return raw_ptr;
     }
 
-    type* type_manager::require_type(const std::string& name) {
+    type* type_manager::require_type(const std::string& name) const {
         if(!type_names.count(name)) throw error("invalid type name");
-        return type_names[name];
+        return type_names.find(name)->second;
     }
 }
diff --git a/src/type_manager.hpp b/src/type_manager.hpp
index 5b45984..71e406e 100644
--- a/src/type_manager.hpp
+++ b/src/type_manager.hpp
@@ -16,7 +16,14 @@ namespace lily {
             type_internal* create_int_type();
             type_internal* create_str_type();
             type_data* create_data_type(const std::string& name);
+            template <typename T, typename ... Args>
+            T* create_type(Args...as) {
+                auto new_type = std::make_unique<T>(as...);
+                T* raw_ptr = new_type.get();
+                types.push_back(std::move(new_type));
+                return raw_ptr;
+            }
 
-            type* require_type(const std::string& name);
+            type* require_type(const std::string& name) const;
     };
 }