The performance of amorphous silicon based solar cells depends on the tailored properties of the various layer materials making up the cell structure as well as on the properties and on the design of the interface regions between the layers. The electronic properties related to the various interfaces are markedly influenced by the Fermi level position within these regions, and by structural properties and chemical compositions resulting from the preparation conditions. Results are presented for the p/i and the TCO/p interfaces and discussed with respect to device performance. Further examples of interface effects are described which are related to chemical reactions and hydrogen diffusion in the course of sample preparation.