We have proposed that learning to count skillfully is initially directed by the implicit knowledge of some counting principles (e.g. Gelman & Gallistel, 1978; Gelman & Meek, 1983). Here we develop the thesis that this is so because principles both set constraints on the nature of counting procedures and provide clues about relevant learning environments. More generally, the idea is that much of early cognitive development proceeds as a function of some domainspecific principles that define domains, focus attention on domain-relevant inputs, and play a central role in the selection and generation of the class of domain-appropriate behaviors. In this chapter, we expand on the notion of principle, taking up both what it does and what it does not entail. In doing so, we hope to clarify the nature of principled or conceptual competence and its relation to the generation of behavior which is governed by procedural competence.