Monoamine oxidases A and B, two isoenzymes encoded by corresponding genes on the X-chromosome, have been implicated in antisocial behavior. This chapter gives an overview of the basic neurobiology of monoamine oxidases, the accumulating evidence on their association with behaviors involving breach of law, and discusses the hypotheses that explain this association. While no mechanistic explanation to such evidence exists, association between MAO variables and antisocial behavior is converging, from studies on both selected groups of participants and population-representative samples, and using different biochemical measures. Common individual variations in activity of both MAO-A and MAO-B seem to shape CNS development at early age and predispose to characteristic behavioral strategies in responding to environmental demands. Hence the role of environment and life events is particularly important in the life course of subjects with the “risk” or “plasticity” variations. MAO variations also highlight gender differences in adaptive strategies.