This chapter provides a systematic review of research published between 2006 and 2022 on the use of artificial intelligence (AI) platforms in writing instruction. We theorize writing and platforms as complex ecologies, investigating the interplay of their relations and implications for educational equity. Our findings suggested three functional categories of AI platforms in writing instruction (assistive, assessment, and authentication) and a focus on the technical dimensions of platforms and their intersections with the cognitive dimensions of writing. Finally, we found a focus on equity notably absent from our corpus. Taken together, these findings suggest an agenda for equity-oriented research and pedagogy that confronts aspects of platform and writing environments that have, to date, been omitted from the empirical record.