A theoretical study of the process of nucleation and growth of crystals in supercooled melts and supersaturated solutions is carried out with allowance for the “diffusion” term in the kinetic equation for the distribution function. The work also takes into account the removal of crystals of a given size from the metastable system, heat outflow (inflow of impurities), and the nucleation frequency is considered in accordance with the Meirs kinetics. Analytical solutions of stationary and nonstationary problems are obtained.