The problems of thin film surface structure, bond strength and type, and electronic features are discussed with respect to metal-metal and metal-insulator layers. It is shown that epitaxial relations are of minor importance in these cases. This is followed by a discussion of the hetero-epitaxial cases, metal-semiconductor and insulator-semiconductor with special reference to silicon on sapphire. Possible measures to improve perfection in the latter case are mentioned. Homo- and hetero-epitaxy in the case of semiconductor-semiconductor layers is treated.