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Although prominence has been given to genes in much biological writing about development, gene expression is such that simple deterministic and additive arguments about genes are usually misconceived. The way in which an individual develops depends on conditions, but the conditions also often depend on the individual. An example is given of how the emerging abilities of a child might result from such an interplay. The development of much individual variation can be explained in terms of relatively simple forms of underlying regulation. Some animal examples are given of how, during development, one of a number of possible outcomes may be selected, as from a jukebox, by environmental conditions. Further cases of regulated interplay between the developing individual and its environment are provided by learning processes, such as imprinting in birds, that appear to occur within sensitive periods in development. Some endogenous control is required, but the self-terminating effects of the learning processes depend on experience. The biological study of behavioural development lays great emphasis on the dynamic character of what happens and in this respect has much in common with other approaches. It is distinctive, though, in that it commonly involves asking what the behavioural processes are for. This functional approach has drawn attention to the special requirements of the young, and the special mechanisms required in the assembly of behaviour. It has also helped understanding of why individuals of the same species, sex and age may differ so much.