The present state of knowledge regarding film formation permits only a qualitative picture of the processes. Reliable data on nucleation and growth are needed for a quantitative comparison between experiment and theory, taking into account the phase of the condensate, the influence of defects in the substrate surface, and so on. Furthermore, a correlation between the results of different methods is necessary for understanding their range of applicability. This information is a prerequisite for controlling film structure as well as the structure-sensitive properties so that film components can be fabricated reliably and reproducibly.