Interest in the roles of hormones rests on the fact that specific behavioural patterns of great biological importance can be regulated by chemical substances produced outside the brain; this discovery, the most important, underlies all subsequent work. Though most studies have been made on sexual behaviour, hormones are also known to be concerned with other categories including aggression, ingestion and, perhaps, adaptive behaviour. Hormones may only be a special case of this phenomenon, since other substances (including those in the diet) may alter intracranial function in a way which is behaviourally significant. Furthermore, it is becoming apparent that events in the environment, including the behaviour of other animals or chemical substances produced by them, may modify the interaction between a hormone and its effects on behaviour. The results of such studies have been applied to humans, though current information on primates suggests important differences between them and the more commonly investigated rodents. A most intriguing discovery has pointed to a role for hormones in a differentiation of sexually dimorphic behaviour, in addition to their activating effect in the adult. This suggests that brief exposure of some hormonally-sensitive part of the brain to a given hormone during a 'critical' period results in permanent alteration in its function. The nature of this change remains obscure though neuroanatomical findings which may be relevant have been described. Understanding of the way hormones alter neural functions is still primitive. Binding of steroid hormones to neural receptors is well known but the functional significance of this remains enigmatic. Interaction of hormones with endogenous cerebral compounds such as monoamines and peptides is only now being studied, though this may prove a turning point for the future.