diff --git a/OOP_exercices/oop_exercices1.php b/OOP_exercices/oop_exercices1.php
new file mode 100644
index 0000000..40b3e9b
--- /dev/null
+++ b/OOP_exercices/oop_exercices1.php
@@ -0,0 +1,102 @@
+// Create a class called Device.
+// The Device has to define the common properties for a device, like, for example,
+// a method called getSerialNumber() which returns the Device serial number.
+
+class Device
+{
+function __construct(int $serialNumber, string $mfr, $firmware)
+{
+$this->serialNumber = $serialNumber;
+$this->mfr = $mfr;
+$this->firmware = $firmware;
+}
+
+function getSerialNumber()
+{
+return $this->serialNumber;
+}
+}
+
+// Then create another one called Mobile and then another one called Tablet.
+// The Mobile and Tablet classes have to extend the Device class and define some particular
+// properties by their own.
+
+class Mobile extends Device
+{
+function __construct(string $system, int $mobileGeneration)
+{
+$this->system = $system;
+$this->mobileGeneration = $mobileGeneration;
+}
+
+function getGeneration()
+{
+$g = $this->mobileGeneration;
+
+if ($g === 5) {
+echo 'Congrats you have 5G! You can go up to 10Gb per second!!!';
+} elseif ($g === 4) {
+echo 'Not bad, with 4G your mobile can go between 100 Mbps and 1Gbps!';
+} else {
+echo 'Please get a new device';
+}
+}
+}
+
+class Tablet extends Device
+{
+function __construct(string $system, bool $connCapacity)
+{
+$this->system = $system;
+$this->mobileCapacity = $connCapacity;
+}
+
+function recogniceSystem()
+{
+if (strtolower($this->system) == 'android') {
+echo 'System detected: you use Android. Good choise!';
+} elseif (strtolower($this->system) == 'apple' || strtolower($this->system) == 'ios') {
+echo "System detected: you use iOS... no comments";
+} else {
+echo "Are you using Windows phone?! What's your problem?!";
+}
+}
+}
+
+$nokia = new Device(1324654, 'Nokia', '13A');
+$xiaomi = new Mobile('Android', 3);
+$mitsubishi = new Tablet('Homebrew', 0);
+
+echo $nokia->getSerialNumber();
+echo '</br>';
+echo $xiaomi->getGeneration();
+echo '</br>';
+echo $mitsubishi->recogniceSystem();
+
+// You should also create a class called DeviceManager. This class has to have a method
+// called getDeviceSerialNumber(Device $device) which has to call the getSerialNumber() method of Device objects.
+// The DeviceManager Instance calls the getDeviceSerialNumber method with each one of the devices objects.
+
+class DeviceManager extends Device
+{
+function __construct(int $serialNumber, string $mfr, $firmware)
+{
+parent::__construct($serialNumber, $mfr, $firmware);
+$this->serial = $serialNumber;
+$this->mfr = $mfr;
+$this->firmware = $firmware;
+$this->getSerialNumber();
+}
+
+
+
+function getDeviceSerialNumber(Device $device)
+{
+$this->getSerialNumber($device);
+}
+}
+
+$motorola = new DeviceManager(1321561, 'Motorola', '1327B');
+echo '</br>';
+echo $motorola->getSerialNumber();
+echo '</br>';
\ No newline at end of file
diff --git a/OOP_exercices/oop_exercices2.php b/OOP_exercices/oop_exercices2.php
new file mode 100644
index 0000000..56a2584
--- /dev/null
+++ b/OOP_exercices/oop_exercices2.php
@@ -0,0 +1,40 @@
+// Create an abstract class Device which defines an abstract method called getDetail($deviceId).
+// Create these two methods getId which returns the name of this device and getSerialNo returns
+// the serial number for this device.
+
+abstract class Device
+{
+function __construct(string $name, int $serial)
+{
+$this->name = $name;
+$this->serial = $serial;
+}
+
+function getId($name)
+{
+return $name;
+}
+
+function getSerialNo($serial)
+{
+return $serial;
+}
+
+abstract function getDetail($deviceId): string;
+}
+
+// Now create a class which extends the Device and implements getDetail($deviceId) method.
+// Each device defines in its own way the details. That’s why it is abstracted to let the subclasses implement this method.
+// The other methods are common between devices.
+
+class Mobile extends Device
+{
+
+function getDetail($deviceId): string
+{
+return "Mobile brand: $this->name</br>Mobile model: $deviceId";
+}
+}
+
+$xiaomi = new Mobile('Xiaomi', 4564651);
+echo $xiaomi->getDetail('RedMi Note');
\ No newline at end of file
diff --git a/OOP_exercices/oop_exercices3.php b/OOP_exercices/oop_exercices3.php
new file mode 100644
index 0000000..a06c2bb
--- /dev/null
+++ b/OOP_exercices/oop_exercices3.php
@@ -0,0 +1,78 @@
+<?php
+
+// Now let’s create an interface for a Device repository.
+// First create a Device class with some properties, as you like.
+
+class Device
+{
+    function __construct(string $brand, int $serial, string $system)
+    {
+        $this->brand = $brand;
+        $this->serial = $serial;
+        $this->system = $system;
+    }
+}
+
+// Then create an interface called DeviceRepository, which defines two methods:
+// - create(Device $device) which has an injection of the Device class you have created
+// - findById($deviceId) which with the $deviceId returns the Device object but remember this is an interface do not implement methods,
+// it just defines them.
+
+interface DeviceRepository
+{
+    public function create(Device $device);
+    public function findById($deviceId);
+}
+
+// Now we need an implementation of the interface DeviceRepository so we are going to create a class called MemoryRepository
+// which implements the DeviceRepository. Here you can define the methods as you like.
+
+class MemoryRepository implements DeviceRepository
+{
+    function __construct()
+    {
+        $this->deviceArr = [];
+    }
+
+    public function create(Device $device)
+    {
+        array_push($this->deviceArr, $device);
+        return $this->deviceArr;
+    }
+
+    public function findById($deviceId)
+    {
+        foreach ($this->deviceArr as $dev) {
+            if ($dev->serial == $deviceId) {
+                return $dev;
+            }
+        }
+    }
+}
+
+$motorola = new Device("Motorola", 134257, "android");
+$x6 = new Device("iPhone", 981321, "ios");
+$nokia = new Device("Nokia", 7461, "windows");
+
+$test = new MemoryRepository();
+$test->create($motorola);
+$test->create($x6);
+print_r($test->create($nokia));
+echo "</br>";
+print_r($test->findById(7461));
+
+// Finally we need to create a class called DeviceManager, which defines a method addDevice(DeviceRepository $repository, Device $device).
+// Create an instance of it and inject to this method the implementation you have done.
+// Remember this is to check your understanding about OOP.
+
+class DeviceManager
+{
+    function addDevice(DeviceRepository $repository, Device $device)
+    {
+        echo "Done";
+    }
+}
+
+$finalTest = new DeviceManager;
+echo "</br>";
+$finalTest->addDevice($test, $nokia);
diff --git a/README.md b/README.md
index e644e6a..5745dbe 100644
--- a/README.md
+++ b/README.md
@@ -12,6 +12,26 @@ In this project you will learn the basics of OOP using mobile devices as a refer
 - [Tools](#tools)
 - [OOP Introduction](#oop-introduction)
 - [Project files](#project-files)
+- [OOP questions](#oop-questions)
+  - [* What is object-oriented programming in general terms?](#-what-is-object-oriented-programming-in-general-terms)
+  - [* What is a class?](#-what-is-a-class)
+  - [* What is an object?](#-what-is-an-object)
+  - [* What is an instance?](#-what-is-an-instance)
+  - [* What is a property?](#-what-is-a-property)
+  - [* What is a method?](#-what-is-a-method)
+  - [* What is the difference between a function and a method?](#-what-is-the-difference-between-a-function-and-a-method)
+  - [* What is a constructor?](#-what-is-a-constructor)
+  - [* What is the difference between a class, an object and an instance?](#-what-is-the-difference-between-a-class-an-object-and-an-instance)
+  - [* What do we understand about the concept of encapsulation?](#-what-do-we-understand-about-the-concept-of-encapsulation)
+  - [* What do we understand about the concept of abstraction?](#-what-do-we-understand-about-the-concept-of-abstraction)
+  - [* What do we understand about the concept of inheritance?](#-what-do-we-understand-about-the-concept-of-inheritance)
+  - [* What do we understand about the concept of polymorphism?](#-what-do-we-understand-about-the-concept-of-polymorphism)
+  - [* What do we understand about the concept of Overload?](#-what-do-we-understand-about-the-concept-of-overload)
+  - [* What do we understand about the concept of Override?](#-what-do-we-understand-about-the-concept-of-override)
+  - [* What differences exist between the concept of Overload and Override?](#-what-differences-exist-between-the-concept-of-overload-and-override)
+  - [* What is a static class?](#-what-is-a-static-class)
+  - [* Look for 3 advantages over object-oriented programming compared to other programming paradigms.](#-look-for-3-advantages-over-object-oriented-programming-compared-to-other-programming-paradigms)
+  - [* Look for disadvantages of this paradigm.](#-look-for-disadvantages-of-this-paradigm)
 
 ## Getting Started
 
@@ -134,3 +154,70 @@ Namespaces are qualifiers that solve two different problems:
 2. They allow the same name to be used for more than one class
 
 In this file you will learn how to create and use namespaces.
+
+## OOP questions
+
+### * What is object-oriented programming in general terms?
+La Programación orientada a objetos es una estructura o paradigma de programación que se basa en la creación de “objetos”, entidades con un estado específico, una identidad y un comportamiento o método.
+
+### * What is a class?
+Una clase es un elemento que establece la estructura general de un tipo de objeto y establece sus métodos y propiedades predeterminadas.
+
+### * What is an object?
+Como ya hemos dicho, un objeto es una entidad o elemento con propiedades, estados y métodos (funciones) propias. 
+
+### * What is an instance?
+Los objetos son instancias de las clases, es decir, una concreción de esa estructura general.
+
+### * What is a property?
+Son los valores que las clases y los objetos tienen, ya sean heredados u obtenidos al crear el objeto. En resumen, son las variables que contienen los objetos desde el inicio.
+
+### * What is a method?
+Los métodos son las funciones que los objetos heredan u obtienen al crearse, tal y como pasa con las propiedades.
+
+### * What is the difference between a function and a method?
+La única diferencia entre ambas es su naturaleza, ya que el método es una función asociada al objeto u clase desde el inicio.
+
+### * What is a constructor?
+El constructor es la función que permite crear un objeto a partir de una clase, estableciendo sus propiedades y métodos.
+
+### * What is the difference between a class, an object and an instance?
+El objeto como elemento general de programación es simplemente un conjunto de datos diferentes y sus funcionalidades.
+Una clase es un elemento que establece la estructura, propiedades y funciones por defecto de los objetos que se podrán crear a partir de la misma.
+La instancia es la concreción de una clase en un objeto.
+
+### * What do we understand about the concept of encapsulation?
+Es la característica de los objetos y de la OOP de recoger un conjunto de datos en un mismo nivel y, juntamente con la característica de ocultación, impide acceder a esos datos fuera del objeto.
+
+### * What do we understand about the concept of abstraction?
+Es la característica de los objetos y de la OOP que permite abstraer o “simplificar” funciones y comportamientos complejos en simples métodos. Es decir, permite crear funcionamientos complejos y ocultarlos al usuario.
+
+### * What do we understand about the concept of inheritance?
+Es la característica de los objetos y de la OOP que permite transmitir información (propiedades y métodos) y estructuras de un elemento a otro, generalmente entre clases o de clases a objetos.
+
+### * What do we understand about the concept of polymorphism?
+Es la característica de los objetos y de la OOP para cambiar y adaptar su forma según los elementos contextuales y los elementos que forman parte del mismo.
+En otras palabras, se pueden adaptar los objetos para que tengan diferentes métodos y resultados aún viniendo de la misma clase.
+
+### * What do we understand about the concept of Overload?
+Es la capacidad de una clase u objeto de tener diferentes métodos con el mismo nombre que necesitan parámetros diferentes y, por lo tanto, responden a los mismos de formas distintas.
+
+### * What do we understand about the concept of Override?
+Es la capacidad de una clase u objeto de redefinir un método de la parent class.
+
+### * What differences exist between the concept of Overload and Override?
+Overload es la adaptación de un método a diferentes parámetros, por lo que los argumentos que le pasemos siempre deberán ser diferentes.
+En cambio override obliga a mantener la misma estructura para el método modificado, siendo los parámetros a pasar siempre los mismos.
+
+### * What is a static class?
+Las clases estáticas son clases que no pueden ser instanciadas ni heredadas, pero si son accesibles (también sus propiedades y métodos) desde fuera de la misma.
+
+### * Look for 3 advantages over object-oriented programming compared to other programming paradigms.
+- Versatilidad en su uso y mantenimiento en programación gracias a su reusabilidad.
+- Su capacidad de protección y privacidad gracias a sus propiedades de encapsulamiento, visibilidad y abstracción.
+- Efectividad al poder usar y adaptar las clases para diferentes usos gracias a la herencia.
+
+### * Look for disadvantages of this paradigm.
+- Tamaño, ya que tanto las clases como sus instancias ocupan espacios de memoria.
+- Performance, a más tamaño menos velocidad en su ejecución (aunque no siempre es así).
+- Complejidad, que tanto le permite construir elementos más complejos y facilitar su uso al usuario, como complicar su creación.