Cognitive development theory examines how we deal with the world and how we coordinate our dealings with it. In this chapter we shall discuss some philosophical problems of such theories, concentrating on Piaget's work since, though much criticized, it is still very influential. His theory is especially interesting philosophically because of its very strong claims about the relation between cognitive development and epistemological questions (see Chapman, 1988), which we will discuss. Also some neo-Piagetian theories will be briefly considered.