AbstractCrystallization – a Thermal Unit Operation. After a definition, this review summarizes important methods of growing single crystals (solution and melt operations), mass crystallization from melts and the gas phase, fractional crystallization (from solution and melts), simple mass crystallization from solution either as cooling, as evaporative, or as vacuum crystallization and necessary equipment, and describes classifying crystallizers. The special crystallization methods described are: reaction crystallization, precipitation, freezing out, adduct crystallization, and zone freezing. Among the peculiarities of crystallization, details are given of the crystal size distribution, habit modifications, Ostwald ripening, the growth rate of the small grain, and prillin