The mechanisms of nucleation and growth of thin films are briefly reviewed. A kinetic theory of the nucleation and growth of two‐dimensional clusters is outlined. Calculations of the nucleation density in a pair binding model are presented and applied to layer plus island Ag deposition systems on W(110), Mo(100), and Si(100) and (111). The behavior at low supersaturation is discussed, and it is shown that explicit treatment of surface vibrations is required to obtain self‐consistent results in this region. Atomic parameters deduced for these Ag deposition systems are tabulated.