This paper deals with the relations and interactions of the substrate, the interface, and the functional hard overlay or diffusion coating deposited by chemical or physical vapor deposition, CVD or PVD, respectively, and how these structures influence the overall mechanical and physicochemical properties of the composite. Emphasis is given to the friction and wear behavior of the refractory carbides, nitrides, borides, and oxides, with and without lubrication, to the characterization of the mechanical strength of the interface as measured by the scratch test and thus, of the adhesion and cohesion properties, and to the corrosion behavior, mainly of TiC.