Variable-temperature scanning tunneling microscopy has been applied to study kinetic processes involved in epitaxial growth. This paper concentrates on nucleation and aggregation of submonolayer Ag films on a Pt(lll) surface. From island density versus temperature data, the activation barrier for Ag adatom diffusion as well as the stability of adsorbed Ag dimers are determined. From the adsorbed aggregate shapes conclusions on Ag perimeter diffusion can be drawn. An anisotropy in edge diffusion leads to dendritic aggregates with the trigonal substrate symmetry. A crossover to randomly ramified fractals is observed upon lowering of the deposition flux.