In this article, the authors take a very conservative view of the contribution of animal models to an understanding of human development. We do not think that homologies can be readily documented with even our most closely related relatives' behavior and psychological functioning. The major contribution of animal models is their provision of food for thought (hypotheses, not facts) about human development and general principles of development, and we describe some of the more significant and interesting of these at length. We also briefly discuss the other applications of animal research toward understanding the development and evolution of behavior, more generally speaking.