This chapter shows that Variation Theory, a way of modeling change in linguistic systems, can help to explain the developmental aspects of first and second language acquisition, and thus serve as part of a transition theory. Perhaps the most studied example of socially patterned variation involves the deletion of the sounds /t/ and /d/ when they occur in a consonant cluster in word final position, so that the words mist and buzzed are pronounced mis' and buzz'. At the time variable rules were introduced, generative grammar had two major goals: to construct an algorithm for generating all and only the grammatical sentences of a language, and to discover principles of Universal Grammar that explained how speakers can learn the grammar. The chapter looks at the psychological underpinnings of variable speech behavior, and turns to some specific studies of variation in the speech of native speakers and language learners.