The amine serotonin has been suggested to play a key role in aggression in many species of animals, including man. Precisely how the amine functions, however, has remained a mystery. As with other important physiological questions, with their large uniquely identifiable neurons, invertebrate systems offer special advantages for the study of behavior. In this article we illustrate that principal with a description of our studies of the role of serotonin in aggression in a lobster model system. Aggression is a quantifiable behavior in crustaceans, the amine neuron systems believed to be important in that behavior have been completely mapped, and key physiological properties of an important subset of these netirons have been defined. These results are summarized here, including descriptions of the "gain-setter" role and "autoinhibition" shown by these neurons. Results of other investigations showing socially modulated changes in amine responsiveness at particular synaptic sites also are described. In addition, speculations are offered about how important developmental roles served by amines like serotonin, which have been well described by other investigators, may be related to the behaviors we are examining. These speculations draw heavily from the organizational/activational roles proposed for steroid hormones by Phoenix et al. (1959).