The demonstration that transitory behavioural responses can be mediated by chemical communication through the secretion of hormones was a turning point of fundamental importance, for it came at a time when it was generally assumed, although sometimes on insecure evidence, that coordination in animals was exclusively the function of the nervous system. No less significant was the later demonstration that hormones are also involved in the regulation of growth, differentiation and metamorphosis. Certain difficulties in identifying and interpreting the sources of these hormones, especially in the invertebrates, were resolved by the development of the concept of neurosecretion. This was another major turning point, which showed the close interrelationship of the nervous and endocrine systems, and thereby revealed common features in the organization of regulatory systems throughout the animal kingdom. Remarkable chemical advances are now enabling us to set hormones in an evolutionary framework which embraces also the chemical communication between members of the same species and between members of ecosystems.