This chapter aims to illustrate the role of working memory in cognitive development that neo-Piagetian theories postulate. It focuses on two of these theories that have elaborated theoretical conceptions about what is working memory and how it develops, namely Case's and Halford's theories. G. S. Halford shares with the other neo-Piagetian authors the idea that the notion of processing capacity can account for observations formerly attributed to stages. Halford assumes that understanding a concept or a situation entails having a mental model of the structure of that concept or situation. R. Case proposed a new theory in which children build structures that permit an executive control of their cognitive and affective experiences by temporally organizing sequences of figurative and operative schemes. The post-Piagetian theories that had been proposed to solve the problems that plagued Piaget's theory did so by focusing on the process of intellectual development rather than its structure, leading to a series of domain-specific theories.