Writing is often seen as having a minor role in second language learning. This article explores recent research that suggests that writing can have a facilitative role in language development. In particular, it focuses on three features of writing: (1) its slower pace, and (2) the enduring record that it leaves, both of which can encourage cognitive processes and interactive moves thought to promote language acquisition, and (3) the need for and the opportunity for greater precision in language use, which may encourage learners to consult their explicit knowledge as they plan, monitor, or review their production. Finally, it provides a critical discussion of the role of explicit knowledge in second language learning and second language writing and the central role of the interface in understanding the relationship between them. (Contains 2 figures.)