The chapter demonstrates the significance of Natural Linguistics from its origins to the latest achievements in the field of first language acquisition. NL is a functionalist framework which validates its claims via sources of external evidence. Language development in a child lies in the heart of the theory. The chapter offers an insight into phonological, morphological, syntactic, discourse, and pragmatic development. First, the strong and weak versions of the acquisition hypothesis are presented. Further, studies of phonological acquisition are reported: the crosslinguistic pre- and proto-morphology/phonology project, case studies of early phonological development, as well as phonotactic development from a crosslinguistic perspective. This is followed by a review of representative research in the morphological domain: the development of grammatical morphology, the role of morphological richness, productivity, and frequency, socio-economic status, the acquisition of extragrammatical morphology, marginal morphology, and morphonotactics. We briefly report on the development of syntax, text, and discourse as well as pragmatics, and end with conclusions and an outlook for future studies.