We analyze the formation of two-dimensional Ag islands following deposition of about 0.1 ML of Ag on Ag(100) over a temperature regime ranging from classical nucleation and growth behavior to almost immobile adatoms, from 300 to 125 K. Particular emphasis is placed on the post-deposition dynamics at the lower end of the temperature range, where the saturation island density is not reached at the end of the deposition, and nucleation and aggregation processes continue with adatoms from the remaining adatom gas. Our analysis combines VT scanning tunneling microscopy experiments with kinetic Monte Carlo simulation of appropriate atomistic models. The only adjustable parameters in the model are the terrace diffusion barrier and prefactor, which can be determined from island density behavior near room temperature. Other processes such as rapid edge diffusion, and “easy” nucleation and aggregation of diagonally adjacent adatoms, are treated as instantaneous. The model excellently reproduces all aspects of behavior at low temperatures, demonstrating that nucleation and growth processes can be described in one consistent scheme, down to the regime of almost immobile adatoms.