This article defines androgens (and anabolic steroids), describes their mechanisms of action, and summarizes their behavioral effects and relevance in animals and humans. A MEDLINE-derived review of the literature on androgens and behavior was performed; pivotal earlier publications were also obtained and included in the review. In animals, the effects of androgens on brain structure and function are well-established and profound, with behavioral implications extending far beyond reproduction. Androgens play a prominent role in the organization or programming of brain circuits, which are subsequently activated by gonadal steroids. In humans, roles for androgens have been described, albeit inconsistently, in the regulation of sexuality, aggression, cognition, emotion, and personality. The relevance of androgens for psychiatry is further suggested by gender-related differences in pharmacokinetics/pharmacodynamics and in the prevalence, course, and treatment response characteristics of several psychiatric disorders. Direct psychoactive effects of exogenously administered androgens have been described for many years, most recently in reports of the psychotoxic effects of anabolic steroids. Data from both animals and humans suggest that the biological and behavioral responses to androgens are context-dependent.