The evolution of the chemical abundance of the ISM and stars in galaxies is driven by the evolution of stars. In this Chapter, we start from the basic physics of stellar structure and review the nucleosynthesis of stars. We introduce stellar nucleosynthesis in details to understand the origin of various heavy elements in galaxies. This gives the starting point of various evolutionary phenomena of galaxies. Then we formulate the chemical evolution of galaxies. First we introduce the most general case, which is fairly complicated because of the finite stellar lifetime. Then we see how the instantaneous recycling approximation and closed-box assumption simplify the governing equations. We derive some fundamental equations for evaluating the observed abundance of heavy elements in galaxies. We do not discuss the element-by-element based chemical evolution in this book. At the end of the chapter, the spectral synthesis of galaxy spectra will be also discussed. The synthesized spectrum is one of the most important tools to examine the evolution of galaxies.