To control the crystal size of a product it is desirable to have a theory based on fundamental science that allows simple modeling and accurate predictions. Four models have been proposed to relate the numbers of crystals formed to experimental parameters. Of these, the classical model (1) is the best known, which focuses on nucleation rate. The primitive (2), Klein-Moisar (3), and Balanced-Nucleation and Growth (4, BNG) models focus on the total number of crystals formed and on crystal size. In the research lab, product development, and manufacturing, the control of crystal size and yield must be related to practical control parameters. These are temperature, crystal solubility, and reactant addition rate. Two other control factors are added active materials that decrease or increase the crystal size for otherwise fixed reaction conditions. For continuous crystallization, the residence time must be added as a control parameter. The four nucleation models will be presented and compared for application in research, product development, and manufacturing.