The paper offers a model of language development, first and second, within a processing perspective. We first sketch a modular view of language, in which competence is embodied in the processing mechanisms. We then propose a novel approach to language acquisition (Acquisition by Processing Theory, or APT), in which development of the module occurs as a natural product of processing activity, without any acquisition mechanisms as such. The approach is illustrated and explicated through examples of the development of content words, derivational morphology, the functional category I with its variable features, and Case and thematic roles, as well as apparent cross-linguistic variation in processing strategies and the status of bootstrapping in the model. We then examine some possible applications to issues in second language acquisition--noticing the gap, the initial state, transfer, and the apparent limits of SLA--and finally offer a broader perspective on the model: its scope, its relations to other approaches, and its possible limits.