The semi-classical description of black holes, as it was originally introduced by Hawking and Bekenstein in the early seventies, where classical solutions of the Einstein equations are coupled to quantum matter fields, opened a window with a glance on the quantum aspects of gravity. This book discusses this semi-classical approach to quantum black holes. It also discuses the philosophical issues about black holes; the Randall-Sundrum II model; the gravitational collapse in alternative theories of gravity; observational limits on modern extended gravity models; and generalized Brans-Dicke models from Kaluza-Klein reductions.