In the previous chapter, Hogan discussed the concept of behaviour systems (cf. Hogan, 1988). In a development of earlier suggestions by Kruijt (1964), Hogan has proposed that behavioural ontogeny involves the development of various kinds of perceptual, motor, and central mechanisms and the formation of connections among them. In this chapter, I evaluate some of the ways in which the neural substrates of developing behavioural mechanisms have been investigated. The chapter is divided into two main parts. First, research aimed at discovering neural localisation of function is discussed, and I provide a brief review of some of the plastic changes at the neuronal level that have been found to occur during the development of specific perceptual mechanisms. In the second half of the chapter, I show how the analysis of neural mechanisms of behaviour can be important for understanding the causal organisation of behaviour during development. That is, neurobiological interventions may enable us to distinguish between, and independently manipulate, different behavioural mechanisms. I discuss a number of different techniques and evaluate what conclusions can be drawn from them as to the organisation of behaviour. We shall see that the results of this research often lead to a further behavioural analysis of perceptual mechanisms. Specific examples are taken mainly from research into the perceptual mechanisms involved in filial imprinting and song learning in birds. As Hogan observes in Chapter 10, in contrast to motor mechanisms, the development of perceptual mechanisms is often dependent on functional experience.