The mechanism of epitaxial growth has been investigated by the study of the nucleation process and early growth stages of single-crystal gold and silver films in (100) and (111) orientations. Films were grown by evaporation on vacuum-cleaved NaCl and mica cleavage planes, and on single-crystal NaCl films deposited on fresh mica cleavage surfaces. In the earliest observable period of growth, nuclei have been found to be as small as 5 Å in size. These nuclei, which contain 10–20 atoms, strongly indicate that growth proceeds continuously from initial nuclei consisting of only a few atoms. In subsequent stages of growth, coalescence processes occur which are governed by mobility of atoms and clusters on the substrate material. Mobility is highest on the evaporated film substrates, and leads to a decrease in nucleation density as a function of time because of rapid coalescence of the mobile nuclei. Nucleation rates have been measured as a function of substrate temperature and activation energies determined for the nucleation process.