Early years practitioners use the Early Years Foundation Stage (EYFS) on a daily basis to plan, observe and assess young children's learning. However, whilst this is a very thorough and broad-ranging curriculum framework, it is not in itself a theory of development or learning, but is, instead, developed out of a range of theoretical perspectives. This chapter explains a few of the more well known of these and their possible application to early years practice. It discusses Development Matters and the Statutory Framework to show the curriculum framework within which practitioners are trying to apply this knowledge. In early years provision, young children's learning and development is measured in terms of their progress towards the Early Learning Goals (ELGs), which are reported on in their EYFS Profile as they come to the end of Reception Class. The chapter considers some of the key theories concerning young children's social and cognitive development and how they might be applied to practioner's practice.