Quantitative measurements of the kinetics of nucleation in some simple substrate‐overgrowth systems have been made using improvedin situelectron microscopy techniques. The nucleation of bismuth and silver on evaporated carbon and the nucleation of bismuth on evaporated SiO substrates were studied as a function of substrate temperature and impinging flux. The results were analyzed in terms of the phenomeno‐logical theory of nucleation and in terms of Walton's atomistic model of condensation from the vapor phase. Both theoretical concepts led to specific conclusions concerning such nucleation parameters as the number of atoms in the critical nucleusn* and the free energy of desorption &Dgr;Gdes. The substrate temperature and impingement flux dependence of the maximum number of deposit particles that can be accommodated on the substrate surface were determined and tentatively interpreted on the basis of nucleation‐limited surface migration. Some nucleation induction time observations were made and were related to the clustering kinetics of adatoms, the growth process of individual nuclei, and the instrumental limitations of the detection method.