The paper deals with the design and operation of cooling and evaporation crystallizers and precipitators for reaction crystallization. It will be shown that the median crystal size L50 of a crystalline product and the Crystal Size Distribution (CSD) mainly depend on the crystallization kinetics, e.g. the rates of nucleation and crystal growth, and that this kinetics is controlled by supersaturation. Therefore, the main objective in crystallization and precipitation is to choose and to maintain the optimal supersaturation with respect to product quality at all times and all locations in the crystallizer or precipitator.