Direct observations of early stages of growth of GaAs on (100)Si are presented. Cross sectional TEM and plan view SEM images show three dimensional island growth, for growth above 300 degree C. Island size, island spacing, surface morphology and stacking fault defect spacing all decrease with substrate temperature for fixed Ga and As//2 fluxes. Below 300C, 7nm thick films are uniform. Diffusion-controlled growth kinetics are inferred.