The external cause which gives us the feeling of hotness or coldness is called heat. Heat is a kind of energy. Temperature is a thermal state that can be measured with thermometer. Heat is the cause while temperature is its consequence. Heat and temperature are important topics both in physics and in our daily life. The branch of physics that deals with heat, mechanical energy and relation between different kinds of energies is called thermodynamics. The main focus of thermodynamics is to establish relationships between different forms of energy. All kinds of energies can be converted into heat energy at particular conditions. Thermodynamics discusses the thermal properties of heavy matters but not tiny particles. In this chapter, we will learn the principle of measuring temperature, first and second laws of thermodynamics, Carnot’s cycle, heat engine, refrigerator and entropy.