Nucleation and growth of Pt on the reconstructed Pt(100)-hex surface was studied by scanning tunneling microscopy. A detailed autocorrelation analysis of island positions reveals direct evidence for strongly anisotropic diffusion, and from the island size distribution, which obeys a simple scaling relation, we conclude that the mobility of dimers is negligible. Finally, kinetic Monte Carlo simulations, incorporating anisotropy in diffusion and finite island sizes, yield new insight into how these two factors affect the island size distribution and the scaling behavior of island density with temperature and deposition rate. [S0031-9007(96)00572-8]