Fluency in a second language is considered important by both learners and teachers, but is not well understood. This paper describes what is known about second language fluency and describes a number of psychological learning mechanisms that might explain how fluency develops. These include the mechanisms underlying the contrast between automatic and controlled processing, the learning mechanisms postulated within Anderson's ACT"' theory of cognition, Bialystok's conception of the control dimension of language development, the notion of restructuring, recent proposals for the redefinition of automaticity as retrieval from memory, both instance and strength versions, and chunking theories. The paper concludes with some suggestions for research into the development of second language fluency itself that can fill gaps in existing knowledge and reduce our dependence on other fields for explanatory principles, while contributing simultaneously to discussion of the mechanisms responsible for skill development in general.The Phenomenon of Fluency ''Fluenf' and "fluency'' are frequently used as non-technical terms and have a number of meanings that should be sorted out. With respect to native language fluency, identified four different things we might have in mind when identifying someone as a particularly fluent speaker. First, we might be thinking of a speaker who easily fills time with talk, a fast talker (Kuiper & Tillis, 1986) such as a disk jockey, a sports announcer, or a conversational partner who hardly lets us get a word in edgewise. Second, we might have the quality of speech more in mind than quantity and might mean by a fluent speaker one whose speech is coherent, complex and dense (Fillmore gives Noam Chomsky as an example of this kind of fluent speaker). We might also consider someone to be especially fluent if they always seem to know the appropriate thing to say in a wide variety of contexts, identifying fluency with pragmatic and affective skills. Finally, Fillmore points out, we may focus on