Throughout the history of the chemical industry, crystals have been produced via crystallization methods that range between something as simple as allowing vats of hot concentrated solution to cool to those as complex as continuous, carefully controlled, multistep processes that yield a crystal product of a particular size or size distribution, shape, moisture content, and purity. This chapter introduces the reader to the concepts and applications of the crystallization process to water desalination and wastewater treatment through discussion of crystallization operations, processes, equipment, describing equations, and design considerations. An application section provides six Illustrative Examples related to the general subject of crystallization.