This paper describes a general theory for the study of electrochemical nucleation on microelectrodes for the case of diffusion-controlled growth. The starting point is the model of Sluyters-Rechbach (M. Sluyters-Rechbach et al., J. Electroanal. Chem. 236 (1987) 1) which is the most general available at present. This model is combined with the approximate solution of Shoup and Szabo (D. Shoup and A. Szabo, J. Electroanal. Chem. 140 (1982) 237) for a chronoamperometric transient on microelectrodes. In this way a single expression is obtained for the description of electrochemical nucleation and growth on microelectrodes for the case of diffusion-controlled overlap. The resulting equation is more general than the equations of Correia et al. (A.N. Correia et al., J. Electroanal. Chem. 407 (1996) 37) for the case of diffusion-controlled growth on inlaid disk electrodes. These authors presented separate equations for the short and long time limits and consider only the limiting cases of instantaneous and progressive nucleation. The limitations of this model and the conditions of its applicability are critically examined. The theory is illustrated with results obtained for the nucleation of copper from concentrated CuSO4 + H2SO4 solutions on platinum microelectrodes in the presence of an organic addition agent (gelatine). Analysis of the experimental transients was performed using a nonlinear least square fitting procedure based on the Levenberg-Marquardt algorithm with the nucleation site density and the nucleation rate constant A as fitting parameters.