It is shown for the special example of silicon films on sapphire and spinel that the ’’interface’’ between both single crystals is more complex than the simple superposition of two different lattice planes. The structural, mechanical, chemical, optical, and electrical aspects of the interface are discussed. It is shown that, depending on the kind of aspect, we can describe it as a two-dimensional interface, a three-dimensional transition layer, or a three-dimensional intermediate layer. All the quoted properties depend both on the kind of preparation of the surface, like polishing or heat treatment, and on the epitaxial deposition process itself, e.g., on temperature and growth rate. A model for this special example of heteroepitaxy will be presented.