A review of research on the development of linguistic competence in second language learners looks at the role played by input to children in their development of linguistic competence, the nature of children's metalinguistic development, and the same processes in the naturalistic second language learning of adults, and then examines the role of a focus on form in second language teaching in developing linguistic competence. Discussion of the development of first-language competence draws on the study of universal grammar and of development of attentional capacities and sensitivity to shifts in input. Children's metalinguistic development is explored through research on metacognition and meta- and epilinguistic awareness. Evidence of the utility of focus on form in assisting adult second language learners in their quest for "fluent accuracy" is considered, and approaches to combining naturalistic language learning with this focus on form are suggested. (Contains 4 tables and 46 references) (MSE)