In this chapter, we look at the physical processes by which stars generate energy. Today we know that stars generate energy through nuclear fusion processes that take place in their inside. Understanding these processes is not only important in order to understand the structure and evolution of stars, but above all to explain the formation and abundance of the various elements found in nature. We begin our discussion with basic considerations about the conditions under which nuclear fusion processes take place and why they can release energy. Then we look at some important reactions in more detail.