Tinbergen (1963) defined ethology as "the biological study of behavior" and, as such, its subject matter is much the same as that of psychology, though its perspective is rather different. From time to time this has led to sharp divisions of opinion between proponents of the two approaches, and the extent to which ethology is relevant to psychology and, in particular, to human behaviour is still a matter of debate. Before considering some of these issues in greater detail, I should like to make a few general comments on changing ideas within ethology and how these relate to the approaches and thinking of psychologists.