Theoretical models of interfaces and misfit defects in poly- and nanocrystalline films are considered. A general classification of interphase boundaries are given, and partly incoherent interfaces are described. The faceting of grain boundaries under misfit stress in thin films is discussed. It is shown that misfit strain can be accommodated by grain boundary dislocations and disclinations, in which case the dislocation and disclination mechanisms of accommodation can act simultaneously under certain conditions. Special attention is paid to the structure and properties of high-Tc superconductor films. We discuss a variety of grain boundary structures and their effect on high-Tc superconductivity. Structural transformations of the grain boundaries are also considered. It is demonstrated that the formation of nanograins with 90° tilt boundaries can be an alternative mechanism of misfit strain accommodation.