This book is an introduction to the study of hormone-behavior interactions, using a comparative approach to explore endocrine mechanisms that have evolved in humans and nonhuman animals. It describes hormone-behavior interactions from a historical perspective, and presents the anatomy, physiology, and biochemistry of the endocrine system, and discusses behavioral sex differences, reproductive and parental behaviors, biological rhythms, homeostatic behaviors, and the role of hormones in learning and memory. The book is aimed at upper-level undergraduate and graduate courses in psychology, biology, and zoology. This second edition increases coverage of molecular and cellular approaches, and contains a new chapter on stress and a revised chapter that addresses affiliation as well as aggression. Material on the use of transgenic mouse models in understanding hormone-behavior interactions, as well as chapter summaries and discussion questions (plus a glossary) is included in this edition.