In this graduation thesis, MQTT protocol which is widely used in IoT systems is described and implemented. MQTT system usually consists of one broker and larger number of clients. MQTT broker is the center of the system. Messages which are sent between the clients have their own topic. First to receive the messages is the MQTT broker. After the messages are received by the broker, messages are filtered and routed according to their topic and are sent to the clients which are subscribed to the corresponding topic. 2 This thesis covers the type of packets which are exchanged over the network by MQTT protocol. Different packet fields along with their values, sizes and purpose are described. DHT11 sensor and ESP32 electronical board are used for the hardware implementation of the MQTT protocol, while for the software implementation Arduino IDE, Node-RED and ThingSpeak have been used. Implementation is displaying the measured sensor values in all three software tools. Arduino IDE is not communicating with the sensor and electronical board over the MQTT protocol, while the Node-RED and ThingSpeak are using MQTT for communication.