This chapter explores the fate of the species-specific language making capacity (LMC) across the lifetimes of individuals and sheds some light on the problem of how it enables them to develop grammatical knowledge about their target languages. As far as the development of a grammatical competence in a first language is concerned, there indeed exists strong evidence in support of the Critical Period Hypothesis (CPH). The more controversial question is whether critical periods also characterize successive acquisition of languages in spite of the fact that the LMC has been activated in the course of first language development. More specifically, the claim is that crucial parts of the language acquisition device (LAD) become inaccessible as a result of neural maturation, and although language acquisition continues to be possible, L2 acquisition differs in a number of crucial ways from L1 development. A grammatical development exhibits a number of sensitive phases.