This book is a pioneer attempt to bridge the gap between the fields of second language acquisition (SLA) and second and foreign language (L2) writing. Its ultimate aim is to advance our understanding of written language learning by compiling a collection of theoretical meta-reflections and empirical studies that shed new light on two crucial dimensions of the theory and research in the field: first, the manner in which L2 users learn to express themselves in writing (the learning-to-write dimension), and, second, the manner in which the engagement in written output practice can contribute to developing competences in an L2 (the writing-to-learn dimension). These two areas of disciplinary inquiry have up until now developed separately: the learning-to-write dimension has been the cornerstone of L2 writing research, whereas the writing-to-learn one has been theorized and researched within SLA studies, hence the relevance of the book for exploring L2 writing-SLA interfaces.