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. These last two chapters have particularly withstood the test of time. 'More than the other parts, ' the author wrote in 1969, 'they show the potential of studying animals in their natural environment, i.e. in the environment that exerts the pressures which each animal species has to meet...I feel very strongly that an...intense effort ought to be made to understand the effects of behaviour; of the ways in which it influences the survival of the species; and that we should try much harder to understand the state of adaptedness and the process of evolutionary adaptation. ' Tinbergen's insights undoubtedly paved the way for significant observational, experimental, and theoretical advances in behavioural ecology and evolution over the past two decades. This book is reissued to make it available to a new generation of researchers and students.