This chapter gives a general conceptual introduction to the educational approach called "Developmental Education", as it was developed in the Netherlands on the basis of Vygotskij's Cultural-historical theory of human development. This educational approach is usually profiled as a play-based curriculum; what this means will be clarified in the course of this chapter. The main foundational concepts will be explained: agency, social situation of development, meaningful learning, involvement, leading activity, zone of proximal development, and finally play will be discussed. The notion of play in particular is presented as a fundamental concept that characterises the whole approach as play-based. This chapter will serve as a reference frame for those that follow.