Direct observations of early stages of molecular-beam epitaxial growth of GaAs on oriented and vicinal (100) Si surfaces are presented. Cross-sectional transmission electron microscopy and plan view scanning electron microscopy images directly reveal three-dimensional island growth for substrate temperatures above 300 °C. Island size, island spacing, surface morphology, and stacking fault defect spacing all increase with substrate temperature for fixed Ga and As fluxes. Below 300 °C, 7-nm-thick films are continuous and uniform. Films deposited on surfaces tilted from (100) coalesce anisotropically with respect to the tilt axis.