Animal studies show clear evidence for a causal link between testosterone and aggression. This review assesses studies involving androgens, principally testosterone, and human aggression. Evidence for a possible effect of prenatal androgens is inconclusive. In adults, higher testosterone levels are found in groups selected for high levels of aggressiveness. Correlations between testosterone and aggression were low when hostility inventories were used, but higher (r = .38) when aggressiveness was rated by others. Regression analysis data and studies of boys at puberty were inconclusive. Other studies show that the outcome of aggressive and competitive encounters can alter testosterone levels, thus confounding interpretation of the correlational evidence. The design of future studies to reveal evidence of a causal link is considered. Suggestions concerning two important methodological problems, the experimental manipulation of hormone levels and the nature of the dependent variable, are made.