The processes of nucleation and growth during electrocrystallization on a foreign substrate are studied both experimentally and theoretically. The most important moments are the mode of nucleation (instantaneous or progressive) and the overlapping of diffusion zones. One of the difficulties is a poor agreement between theoretically computed number of growing nuclei (based on experimental potentiostatic transient curves) and experimentally (microscopically) observed number of clusters.   The actual conditions of overlap are more complex than it was supposed at the development of the theory. By this reason it is unlikely that the extreme points I* and t* can be used for the calculations; the initial regions of the curves are more informative. Moreover, the theory does not take into account the overall shape of I – t dependence for the unique cluster. But both the current density in the maximum and the time required for reaching it depend in a complicated manner upon the balance between diffusion coefficient, exchange current density, overvoltage, A and N0. The position of the maximum is useless for the determination of A and N0 because these two qualities are present everywhere as their product.   The peculiarity of our model is that the hemispherical diffusion presents simultaneously with the developing planar front. After the overlap of depleted zones clusters continue to grow for a long time without merging, and hemispherical diffusion to them, along with planar one, persists in existing. In other words, the overall overlapping of diffusion zones leads to developing of planar diffusion, but each cluster continues its growth over a long period under hemispherical conditions. So there is no “gradual replacing” of one type of diffusion by another, but their coexistence.   We have obtained the solution of a problem concerning the radius of a depleted zone around the growing cluster at diffusion kinetics. It is shown that the reduction of the nucleation probability an each instant depends both on diffusion coefficient and on overpotential as well as on individual properties of a metal, i. e. its surface energy and molar volume.     The results allow determine the overall number of clusters formed using the values of AN0 and D. The calculated values are close to the experimental data. The shape of the transition curve is not uniquely determined by instantaneous or progressive type of the nucleation. However the course of the curve gives a possibility to find the exchange current density, nucleation rate, diffusion coefficient and the total number of clusters. We do understand that the weak point of the described model is the linear concentration dependence at which the additivity of the resistances is based. We hope to correct this defect in future.