diff --git a/CMakeLists.txt b/CMakeLists.txt
index 67e3f8e..1aaea1c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,19 +2,36 @@ cmake_minimum_required(VERSION 3.2)
 
 set(CMAKE_CXX_STANDARD 14)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
+#set(CMAKE_CXX_COMPILER g++-10)
 
 project(Cars)
+include(FetchContent)
+FetchContent_Declare(
+  googletest
+  GIT_REPOSITORY https://github.com/google/googletest.git
+  GIT_TAG        main
+)
+# For Windows: Prevent overriding the parent project's compiler/linker settings
+set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(googletest)
 
 set(SRC_LIST
+    Car.cpp
     ElectricCar.cpp
     ElectricEngine.cpp
     HybridCar.cpp
-    main.cpp
     PetrolCar.cpp
     PetrolEngine.cpp
     Car.cpp
 )
 
-add_executable(${PROJECT_NAME} ${SRC_LIST})
-target_compile_options(${PROJECT_NAME} PUBLIC -Wall -Wpedantic -Wextra)
-target_include_directories(${PROJECT_NAME} PUBLIC ${CMAKE_SOURCE_DIR})
+add_executable(${PROJECT_NAME} ${SRC_LIST} main.cpp)
+
+#target_compile_options(${PROJECT_NAME} PUBLIC -Wall -Wpedantic -Wextra)
+#target_include_directories(${PROJECT_NAME} PUBLIC ${CMAKE_SOURCE_DIR})
+
+add_executable(${PROJECT_NAME}-ut ${SRC_LIST} test.cpp)
+target_link_libraries(${PROJECT_NAME}-ut gtest_main)
+
+enable_testing()
+add_test(NAME ${PROJECT_NAME}_test COMMAND ${PROJECT_NAME})
diff --git a/Car.cpp b/Car.cpp
index 945e888..64ce3aa 100644
--- a/Car.cpp
+++ b/Car.cpp
@@ -3,5 +3,22 @@
 
 void Car::turnLeft()      { std::cout << __FUNCTION__ << std::endl; }
 void Car::turnRight()     { std::cout << __FUNCTION__ << std::endl; }
-void Car::brake()         { std::cout << __FUNCTION__ << std::endl; }
-void Car::accelerate(int) { std::cout << __FUNCTION__ << std::endl; }
+
+void Car::brake() {
+     speed_ = 0.0; 
+}
+void Car::accelerate(int speed) { 
+    if(speed < 0.0) {
+        throw std::range_error("Speed can't be below zero!");
+    }
+    if(speed > maxSpeed) {
+        throw std::range_error("Speed can't be over 300.0 km/h!");
+    }
+    speed_ = speed;
+}
+void Car::changeEngine(std::unique_ptr<PetrolEngine>) {
+    throw std::invalid_argument("This is not a petrol car!");
+}
+void Car::changeEngine(std::unique_ptr<ElectricEngine>) {
+    throw std::invalid_argument("This is not an electric car!");
+}
diff --git a/Car.hpp b/Car.hpp
index 4c8fd42..d25e051 100644
--- a/Car.hpp
+++ b/Car.hpp
@@ -1,12 +1,33 @@
 #pragma once
+#include <exception>
+#include <memory>
+#include "ElectricEngine.hpp"
+#include "PetrolEngine.hpp"
+
+constexpr float maxSpeed = 300.0;
 
 class Car
 {
 public:
+    virtual ~Car(){}
+    
     void turnLeft();
     void turnRight();
     void brake();
     void accelerate(int speed);
     virtual void refill() = 0;
-    virtual ~Car(){}
+    virtual void changeEngine(std::unique_ptr<PetrolEngine> enginePtr);
+    virtual void changeEngine(std::unique_ptr<ElectricEngine> enginePtr);
+    
+    // getters
+    float getSpeed() const { return speed_; }
+    
+    // execption
+    class CarMoving : public std::logic_error {
+        public:
+            CarMoving(const std::string &msg) :
+                std::logic_error(msg) {}
+    };
+private:
+    float speed_;
 };
diff --git a/ElectricCar.cpp b/ElectricCar.cpp
index 0739201..51647ca 100644
--- a/ElectricCar.cpp
+++ b/ElectricCar.cpp
@@ -1,17 +1,20 @@
 #include "ElectricCar.hpp"
 #include <iostream>
 
-ElectricCar::ElectricCar(ElectricEngine* engine)
-    : engine_(engine)
+ElectricCar::ElectricCar(std::unique_ptr<ElectricEngine> engine)
+    : engine_(std::move(engine))
 {
     std::cout << __FUNCTION__ << std::endl;
 }
 
 ElectricCar::~ElectricCar()       { std::cout << __FUNCTION__ << std::endl; }
 void ElectricCar::charge()        { std::cout << __FUNCTION__ << std::endl; }
-void ElectricCar::changeEngine(ElectricEngine* ee) {
-    delete engine_;
-    engine_ = ee;
+void ElectricCar::changeEngine(std::unique_ptr<ElectricEngine> electricEngine) {
+    if(this->getSpeed() != 0.0) {
+        engine_ = std::move(electricEngine);
+        return;
+    }
+    throw Car::CarMoving{"Can't change an engine when car is moving!"};
 }
 void ElectricCar::refill() {
     charge();
diff --git a/ElectricCar.hpp b/ElectricCar.hpp
index e57fd3d..c7d0692 100644
--- a/ElectricCar.hpp
+++ b/ElectricCar.hpp
@@ -5,13 +5,13 @@
 class ElectricCar : public virtual Car
 {
 public:
-    ElectricCar(ElectricEngine* engine);
+    ElectricCar(std::unique_ptr<ElectricEngine> engine);
     ~ElectricCar();
 
-    void changeEngine(ElectricEngine* ee);
+    void changeEngine(std::unique_ptr<ElectricEngine> electricEngine);
     void refill() override;
 
 private:
     void charge();
-    ElectricEngine* engine_;
+    std::unique_ptr<ElectricEngine> engine_;
 };
diff --git a/HybridCar.cpp b/HybridCar.cpp
index bcb0e09..c061786 100644
--- a/HybridCar.cpp
+++ b/HybridCar.cpp
@@ -1,9 +1,9 @@
 #include "HybridCar.hpp"
 #include <iostream>
 
-HybridCar::HybridCar(PetrolEngine* petrolEng, ElectricEngine* electricEng)
-    : ElectricCar(electricEng)
-    , PetrolCar(petrolEng)
+HybridCar::HybridCar(std::unique_ptr<PetrolEngine> petrolEng, std::unique_ptr<ElectricEngine> electricEng)
+    : ElectricCar(std::move(electricEng))
+    , PetrolCar(std::move(petrolEng))
 {
     std::cout << __FUNCTION__ << std::endl;
 }
@@ -12,4 +12,5 @@ HybridCar::~HybridCar()       { std::cout << __FUNCTION__ << std::endl; }
 void HybridCar::refill() {
     ElectricCar::refill();
     PetrolCar::refill();
-}
\ No newline at end of file
+}
+
diff --git a/HybridCar.hpp b/HybridCar.hpp
index 18876cd..6d1ded6 100644
--- a/HybridCar.hpp
+++ b/HybridCar.hpp
@@ -8,7 +8,7 @@
 class HybridCar : public ElectricCar, public PetrolCar
 {
 public:
-    HybridCar(PetrolEngine* petrolEng, ElectricEngine* electricEng);
+    HybridCar(std::unique_ptr<PetrolEngine> petrolEng, std::unique_ptr<ElectricEngine> electricEng);
     ~HybridCar();
     void refill() override;
 };
diff --git a/InvalidGear.hpp b/InvalidGear.hpp
new file mode 100644
index 0000000..d4b8920
--- /dev/null
+++ b/InvalidGear.hpp
@@ -0,0 +1,6 @@
+#pragma once
+#include <stdexcept>
+
+struct InvalidGear : public std::logic_error {
+    InvalidGear(std::string msg) : std::logic_error(msg) {}
+};
\ No newline at end of file
diff --git a/PetrolCar.cpp b/PetrolCar.cpp
index 12ddc07..1d04b39 100644
--- a/PetrolCar.cpp
+++ b/PetrolCar.cpp
@@ -1,18 +1,26 @@
 #include "PetrolCar.hpp"
 #include <iostream>
 
-PetrolCar::PetrolCar(PetrolEngine* engine)
-    : engine_(engine)
+PetrolCar::PetrolCar(std::unique_ptr<PetrolEngine> engine)
+    : engine_(std::move(engine))
 {
     std::cout << __FUNCTION__ << std::endl;
 }
 
 PetrolCar::~PetrolCar()         { std::cout << __FUNCTION__ << std::endl; }
 void PetrolCar::refuel()        { std::cout << __FUNCTION__ << std::endl; }
-void PetrolCar::changeEngine(PetrolEngine* pe) {
-    delete engine_;
-    engine_ = pe;
+
+void PetrolCar::changeGear(int gear) {
+    engine_->changeGear(gear);
+}
+void PetrolCar::changeEngine(std::unique_ptr<PetrolEngine> petrolEngine) {
+    if(this->getSpeed() != 0.0) {
+        engine_ = std::move(petrolEngine);
+        return;
+    }
+    throw Car::CarMoving{"Can't change an engine when car is moving!"};
 }
+
 void PetrolCar::refill() {
     refuel();
 }
diff --git a/PetrolCar.hpp b/PetrolCar.hpp
index f03c382..37566bf 100644
--- a/PetrolCar.hpp
+++ b/PetrolCar.hpp
@@ -1,4 +1,6 @@
 #pragma once
+#include <memory>
+#include <utility>
 #include "PetrolEngine.hpp"
 #include "Car.hpp"
 
@@ -6,13 +8,14 @@
 class PetrolCar : virtual public Car
 {
 public:
-    PetrolCar(PetrolEngine* engine);
+    PetrolCar(std::unique_ptr<PetrolEngine> petrolEngine);
     ~PetrolCar();
 
-    void changeEngine(PetrolEngine* pe);
+    void changeGear(int gear);
+    void changeEngine(std::unique_ptr<PetrolEngine> petrolEngine);
     void refill() override;
 
 private:
     void refuel();
-    PetrolEngine* engine_;
+    std::unique_ptr<PetrolEngine> engine_;
 };
diff --git a/PetrolEngine.cpp b/PetrolEngine.cpp
index 2aeceb9..863344e 100644
--- a/PetrolEngine.cpp
+++ b/PetrolEngine.cpp
@@ -1,5 +1,7 @@
 #include "PetrolEngine.hpp"
 #include <iostream>
+#include <string>
+
 
 PetrolEngine::PetrolEngine(int power, float capacity, int gears)
     : power_(power)
@@ -15,6 +17,13 @@ void PetrolEngine::changeGear(int gear)
     // TODO: Add checking if gear is between -1 and gears_
     // -1 is for REAR
     // 0 is for NEUTRAL
+    if(gear < -1 || gear > gears_) {
+        throw InvalidGear("Invalid gear. Gear needs to be between 1 and " + std::to_string(gear));   
+    }
+    if(gear != 0 && abs(gear - currentGear_) != 1) {
+        throw InvalidGear("Invalid gear change from: " + std::to_string(currentGear_)+ " to: " + std::to_string(gear));
+    }
     currentGear_ = gear;
+
     std::cout << __FUNCTION__ << std::endl;
 }
diff --git a/PetrolEngine.hpp b/PetrolEngine.hpp
index 0996576..83a4747 100644
--- a/PetrolEngine.hpp
+++ b/PetrolEngine.hpp
@@ -1,8 +1,10 @@
 #pragma once
+#include "InvalidGear.hpp"
 
 class PetrolEngine
 {
 public:
+
     PetrolEngine(int power, float capacity, int gears);
     void changeGear(int gear);
 
diff --git a/main.cpp b/main.cpp
index 400296a..3fbecd7 100644
--- a/main.cpp
+++ b/main.cpp
@@ -6,29 +6,24 @@
 
 int main()
 {
+    // std::cout << std::endl << "OPEL" << std::endl;
+    // PetrolCar opel(new PetrolEngine(120, 1800, 6));
+    // opel.accelerate(50);
+    // opel.brake();
+    // opel.accelerate(-900);
+    // opel.refuel();
 
-    std::cout << std::endl << "OPEL" << std::endl;
-    PetrolCar opel(new PetrolEngine(120, 1800, 6));
-    Car* car = &opel;
-    car->accelerate(50);
-    car->brake();
-    car->accelerate(-900);
-    car->refill();
-    // car->changeEngine(new PetrolEngine(150, 700, 7));
-
-    std::cout << std::endl << "NISSAN" << std::endl;
-    ElectricCar nissan(new ElectricEngine(130, 650));
-    car = &nissan;
-    car->refill();
-    car->accelerate(80);
+    // std::cout << std::endl << "NISSAN" << std::endl;
+    // ElectricCar nissan(new ElectricEngine(130, 650));
+    // nissan.charge();
+    // nissan.accelerate(80);
     // nissan.engine_ = new ElectricEngine(150, 700);  // Changing an engine during driving is not safe
-    // car->changeEngine(new ElectricEngine(150, 700));
-    car->turnLeft();
+    // nissan.turnLeft();
 
-    std::cout << std::endl << "TOYOTA" << std::endl;
-    HybridCar toyota(new PetrolEngine(80, 1400, 5), new ElectricEngine(100, 540));
-    car = &toyota;
-    car->accelerate(100);
-    car->brake();
-    car->refill();
+    // std::cout << std::endl << "TOYOTA" << std::endl;
+    // HybridCar toyota(new PetrolEngine(80, 1400, 5), new ElectricEngine(100, 540));
+    // toyota.accelerate(100);
+    // toyota.brake();
+    // toyota.charge();
+    // toyota.refuel();
 }
diff --git a/test.cpp b/test.cpp
new file mode 100644
index 0000000..9698951
--- /dev/null
+++ b/test.cpp
@@ -0,0 +1,52 @@
+#include "gtest/gtest.h"
+#include "Car.hpp"
+#include "PetrolCar.hpp"
+#include "ElectricCar.hpp"
+#include "HybridCar.hpp"
+#include "InvalidGear.hpp""
+
+
+// Tests throwing std::range_error when accelerate to speed = -999 or speed > maxSpeed is invoked
+TEST(PetrolCarClassTest, ShouldThrowRangeError) {
+    PetrolCar opel(std::make_unique<PetrolEngine>(120, 1800, 6));
+    EXPECT_THROW(opel.accelerate(-999), std::range_error);
+    EXPECT_THROW(opel.accelerate(350), std::range_error);
+
+}
+// Tests throwing InvalidGear when wrong gear is set
+TEST(PetrolCarChangeGearTest, ShouldThrowIvalidGear) {
+    PetrolCar opel(std::make_unique<PetrolEngine>(120, 1800, 6));
+    EXPECT_THROW(opel.changeGear(-2), InvalidGear);
+    EXPECT_THROW(opel.changeGear(7), InvalidGear);
+    EXPECT_THROW(opel.changeGear(2), InvalidGear);
+    opel.changeGear(1);
+    EXPECT_THROW(opel.changeGear(5), InvalidGear);
+}
+// Tests if changeGear doesn't throw an exception
+TEST(PetrolCarChangeGearTest, ShouldNotThrowException) {
+    PetrolCar opel(std::make_unique<PetrolEngine>(120, 1800, 6));
+    EXPECT_NO_THROW(opel.changeGear(1));
+    EXPECT_NO_THROW(opel.changeGear(2));
+    EXPECT_NO_THROW(opel.changeGear(3));    
+    EXPECT_NO_THROW(opel.changeGear(2));
+    EXPECT_NO_THROW(opel.changeGear(1));
+    EXPECT_NO_THROW(opel.changeGear(0));
+    EXPECT_NO_THROW(opel.changeGear(-1));
+}
+// Tests throwing std::range_error when accelerate to speed = -999 or speed > maxSpeed is invoked
+TEST(ElectricCarClassTest, ShouldThrowRangeError) {
+    ElectricCar nissan(std::make_unique<ElectricEngine>(130, 650));
+    EXPECT_THROW(nissan.accelerate(-999), std::range_error);
+    EXPECT_THROW(nissan.accelerate(350), std::range_error);
+}
+// Tests throwing std::invalid_argument when changing engine for the wrong one
+TEST(CarClassPointerTest, ShouldThrowInvalidArgument) {
+    ElectricCar nissan(std::make_unique<ElectricEngine>(130, 650));
+    Car * car = &nissan;
+    // Electric car can't have petrol engine
+    EXPECT_THROW(car->changeEngine(std::make_unique<PetrolEngine>(150, 700, 7)), std::invalid_argument); 
+    PetrolCar opel(std::make_unique<PetrolEngine>(120, 1800, 6));
+    car = &opel;
+    // Petrol car can't have electric engine
+    EXPECT_THROW(car->changeEngine(std::make_unique<ElectricEngine>(130, 650)), std::invalid_argument);
+}
\ No newline at end of file