In 1963, a landmark paper by Niko Tinbergen laid out the aims and methods of ethology and, in so doing, extended and clarified Julian Huxley's classification of the different ways in which one can investigate biological processes. I discuss the status of one of these "four Why questions", that of function or survival value, and the relationship of Tinbergen's ethology to behavioural ecology, the main field asking functional questions about animal behaviour today. Function itself can be defined in many different ways and behavioural ecologists themselves use it both in the context of current utility and selective history. I review these definitions in the light of analyses by philosophers of science, behavioural ecologists and, of course, Tinbergen's own use of the word. I defend the view, accepted by many philosophers of science, that the definition of 'function' must have a historical component, both to avoid teleology and to retain the everyday sense of questioning 'What is it for?' That said, in reviewing the different methods that can be used to determine function, I defend the view that investigations of current utility, as practised by behavioural ecologists, can provide the most important clues to the selective forces that have shaped behaviours. Finally, I consider the evolution of the discipline of behavioural ecology, its current status and future prospects.