Outside the core grammar, the set of "peripheral" or marked properties of a language include exceptions or relaxations of the settings of core grammar and the idiosyncratic features of the language governed by particular lexical items. The core/peripheral distinction has direct implications for grammatical development in children. The effects of the core/periphery distinction in actual language acquisition are explored, and it is suggested that this theoretical distinction explains a number of properties of real-time acquisition. It is demonstrated that core and peripheral aspects of grammar are acquired differently and that the latter poses a more substantial learning problem for the child. The evidence considered comes from the acquisition of inflectional morphology. Acquisition of verbal inflection is discussed, proposing that the variability in the grammatical status of inflection across languages accounts for its relative ease or difficulty of acquisition. The second section addresses markedness and acquisition, drawing on evidence from agrammatic aphasics that supports the analysis of inflection proposed earlier. (MSE)