Niko Tinbergen (1963) put behavioural development on the map as one of the four main problems in behavioural biology. Developmental research at the time was still in the grip of the nature/nurture debate. In his discussion, Tinbergen advocated an interactionist approach to development, which has been the main point of view in developmental research since then. In this paper, we review research in a number of different areas, including imprinting, song learning, motivational systems, human language, and attachment. It has become clear that sensitive periods are important in all these areas, and that some aspects of development are generally irreversible; but these phenomena are not as rigid as was thought previously. Similarly, predispositions, or biases, play an important role in perceptual development, but here too there is much more flexibility than is sometimes suggested. Learning mechanisms underlying imprinting and song acquisition are very similar to those underlying other forms of associative learning. Cognitive concepts such as neural representations and behaviour systems are becoming more common in the analysis of developmental questions. Finally, there are numerous parallels between developmental processes in humans and other animals.