The growing occurrence of heterogeneous materials such as composites or coated substrates in structural parts makes it necessary for designers and scientists to deal with the specific features of the mechanical behavior of solid interfaces. This book introduces basic concepts on mechanical problems related to the presence of solid/solid interfaces and their practical applications. The various topics discussed here are the mechanical characterization of interfaces, the initiation and growth of cracks along interfaces, the origin and control of interface adhesion, focusing in particular on thin films on substrate systems. It is designed and structured to provide a solid background in the mechanics of heterogeneous materials to help students in materials science, as well as scientists and engineers.