Ethologist and Nobel laureate Niko Tinbergen laid the foundations for the scientific study of animal behaviour when he formulated its four main problems: evolution, function, development and causation. Celebrating the fortieth anniversary of the publication of Tinbergen's classic article 'On aims and methods of ethology', in this book an international cast of leading behavioural biologists reflect on the enduring significance of his groundbreaking proposals. Following a reprint of Tinbergen's seminal paper on the famous 'four why's', a contemporary perspective is presented on each of the four problems. In addition, two essays discuss the wider significance of recent trends in evolutionary psychology and neuroecology tointegrate the four why's. This wide ranging book, with a foreword by Aubrey Manning, will appeal to students and researchers in behavioural biology, experimental psychology and cognitive neuroscience.