Published data on the effects of androgen deficiency and testosterone administration on body composition in men and women are reviewed. In experimental paradigms, androgen deprivation decreases lean body mass and increases fat mass in men, and physiologic replacement reverses these abnormalities. The anabolic effects of testosterone administration on muscle in men are well-established, and current understanding of the underlying mechanisms are discussed. Randomized, placebo-controlled studies have been performed to investigate the effects of testosterone administration on body composition in a number of male hypogonadal states, including HIV-associated weight loss, supraphysiologic glucocorticoid administration, aging and obesity, with variable outcomes, and the results are reviewed. There are few data investigating the effects of hypoandrogenemia or androgen replacement on body composition in women, in whom endogenous testosterone levels are a fraction of those in men. A recent randomized, placebo-controlled study of physiologic testosterone replacement therapy in women with profound hypoandrogenemia due to hypopituitarism demonstrated an increase in skeletal muscle mass but no change in body fat. Further research is needed to establish the effects of endogenous androgens on the regulation of body composition in women.