The serotonergic system in the CNS has complex interactions with many, if not all other neurotransmitter systems in the brain. Its localization, distribution and amazing receptor diversity makes it an appealing system for modulatory aspects in many basic behaviours, including food and water intake, sexual behaviour and aggression. Notwithstanding decades of research into the putative role of the serotonergic system in aggression, no clear picture about its specific role has emerged. It seems, dependent on state or trait, to be involved in either the performance or the termination of aggressive behaviours. The present technology appears not developed enough to give answers to these questions. Application of drugs and particular selective ligands for certain subtype receptors seems a more promising approach to unravelling the role of 5-HT in aggression. The (postsynaptic) 5-HT1B and to a lesser extent, the 5-HT1A receptor seems to play a prominent role, at least in rodents, in the modulation of (offensive) aggression.