Different nucleation and growth mechanisms (Stranski-Krastanov and Frank-van der Merwe) were found to operate in many cases of electrocrystallization and thin film formation of metals Me on foreign substrates, S, in systems with underpotential deposition (upd) phenomena. Modern in situ techniques, particularly in situ STM with lateral atomic resolution, have directly shown that two-dimensional upd adlayers on S act as precursors for the subsequent nucleation and growth of three-dimensional Me bulk phase. This is reflected in the nucleation kinetics and epitaxial orientation of thin film deposits. The growth mode determines the microstructure and properties of the deposit. Typical examples for the Stranski-Krastanov and Frank-van de Merwe growth mechanisms were observed in the systems Ag(hkl)/Pb2+ and Au(hkl)/Ag+, respectively. Ultra-thin Me-films and heterostructures (sandwich-structured Memonolayers and surface alloys) can be deposited in the upd and opd ranges. In systems, where the Frank-van der Merwe mechanism operates, the thickness of a thin Me film can be varied between one monolayer and a finite number of monolayers. The thickness, structure and composition of heterostructures can be controlled by the substrate, the composition of the electrolyte, and the experimental conditions used. The systems Ag(hkl)/Pb2+, Tl+ and Au(hkl)/Ag+, Pb2+, Tl+ are discussed as typical examples. The results obtained are of importance for the nanotechnology dealing with structuring and modification of metal surfaces in the nanometer range in vertical and lateral direction.