Recent investigations in neurogenomics have opened up new lines of research into a crucial genetic problem—the pathway from genes to behavior. This paper concentrates on the involvement of protein elements in the brain neurotransmitter serotonin (5‐HT) system in the genetic control of aggressive behavior. Specifically, it describes: (1) the effect of the knockout of MAO A, the principal enzyme in 5‐HT degradation, (2) the association of intermale aggression with the polymorphism in the Tph2 gene encoding the key enzyme in 5‐HT (...) synthesis in the brain, tryptophan hydroxylase (TPH), and (3) the effect of selective breeding for nonaggressive behavior on 5‐HT metabolism, TPH activity and 5‐HT1A receptors in the brain. The review provides converging lines of evidence that: (1) brain 5‐HT contributes to a critical mechanism underlying genetically defined individual differences in aggressiveness, and (2) genes encoding pivotal enzymes in 5‐HT metabolism (TPH and MAO A), 5‐HT‐transporter, 5‐HT1A and 5‐HT1B receptors belong to a group of genes that modulate aggressive behavior. BioEssays 28: 495–503, 2006. © 2006 Wiley Periodicals, Inc. (shrink)