This chapter develops the framework for understanding the excitation of charge carriers in semiconductors by interaction with light. The definition of the photogeneration rate is followed by derivations of the optical absorption coefficient for interband transitions in direct and indirect bandgap semiconductors. The special case of light absorption in highly doped and defective semiconductors is addressed by introducing the concepts of the Burstein-Moss shift and of Urbach tails. Finally, optical excitation is considered beyond the one-electron approximation by analyzing absorption spectra in 3D and 2D excitonic materials. Following examples of the exciting prospects offered by excitons for computing and for accessing Bose-Einstein condensates, the chapter concludes with a description of practical methods in which optical absorption measurements are used to determine the bandgap of semiconductors.