Behavioural ecologists and evolutionary biologists have long recognised Professor Tinbergen's great prescience in placing the study of animal behaviour firmly in an ecological and evolutionary context nearly fifty years ago. This is a reprint of the 1969 edition of The Study of Instinct (originally published in 1951). The first six chapters cover behaviour as a response to stimuli, the neurophysiological bases of innate behaviour as then understood, and the development of behavioural patterns in individuals. The final two chapters are devoted to the adaptativeness of behaviour and evolutionary aspects of behaviour.