Polymorphism is frequently encountered in many pharmaceutical, chemical, and food products. Polymorphs have the same chemical composition but different crystal structures, and therefore differ in their physicochemical properties such as stability, solubility, and bioavailability. These property differences can influence the product performance. Thus, the production of specific and well-defined polymorphs is crucial, especially in the pharmaceutical industry. This thesis focuses on how to establish control over the polymorph formation. Crystallization kinetics, especially nucleation kinetics and thermodynamics, were studied to improve the understanding of polymorphic crystallization behavior. Using the improved fundamental understanding, control over the polymorphism of model organic compounds has been successfully established.