Given the importance of organic crystals in a wide range of industrial applications, the chemistry, biology, materials science, and chemical engineering communities have focused considerable attention on developing methods to control crystal structure, size, shape, and orientation. Tailored additives have been used to control crystallization to great effect, presumably by selectively binding to particular crystallographic surfaces and sites. However, substantial knowledge gaps still exist in the fundamental mechanisms that govern the formation and growth of organic crystals in both the absence and presence of additives. In this review, we highlight research discoveries that reveal the role of additives, either introduced by design or present adventitiously, on various stages of formation and growth of organic crystals, including nucleation, dislocation spiral growth mechanisms, growth inhibition, and nonclassical crystal morphologies. The insights from these investigations and others of their kind are likely to guide the development of innovative methods to manipulate crystallization for a wide range of materials and applications.