This chapter provides an overview of the current state of research on the role of writing in second language (L2) learning. For many years L2 writing and second language acquisition (SLA) research developed independently. However, recent interest in the learning potential of writing and writing-related processes has led to a prolific area of research investigating L2 writing from a writing-to-learn perspective. This chapter, a novel addition to the second edition of the handbook, acknowledges this research and responds to calls for a more central role of L2 writing in SLA research agendas. Scholarly work on the learning affordances of L2 writing has examined both individual and collaborative writing, as well as the use and processing of written corrective feedback. The chapter discusses the theoretical underpinnings of research in each of these areas and provides a summary of key issues and findings. It reviews selected empirical studies, involving a range of learner populations and L2s, that have contributed to our understanding of how and why writing and writing-related processes facilitate L2 learning. The chapter concludes with a forward look, pointing out directions for future research.