This book provides an elementary introduction to the study of animal behaviour, aimed primarily at sixth formers and undergraduates attending short courses in the subject. It introduces the basic ideas and concepts of modern ethology set in a historical context, thus showing how views have changed since the simple theories put forward by the founders of the field, such as Lorenz and Tinbergen, 30 years or more ago. The book is not intended to be comprehensive, nor could it be at this length, but it concentrates on putting across the basic principles of the subject as briefly and lucidly as possible. It does this with the aid of carefully selected examples, some recent and others classics in the field, and with numerous illustrations.