The study of planning in second language (L2) writing research is heavily influenced by two research domains: (a) early research on cognition in first language (L1) composing processes and (b) second language acquisition (SLA) research. The first research domain has been instrumental in determining the specific systems and processes involved in composing and has led to widely accepted models of L1 writing (Bereiter & Scardamalia, 1987*; Flower & Hayes, 1980*; Hayes, 1996, 2012) as well as a widely accepted model of the interaction between working memory and L1 writing systems (Kellogg, 1996*; Kellogg, Whiteford, Turner, Cahill, & Mertens, 2013). The influence of these early studies is still felt in process approaches to composition instruction commonly implemented in L1 and L2 writing classes. The second research domain—SLA and more specifically task-based language teaching/learning—has come to view planning as a feature of task complexity that can be manipulated to facilitate the production of language that is complex (syntactically and/or lexically), accurate, and/or fluent (Robinson, 2011*; Skehan, 1998*; Skehan & Foster, 2001). This research timeline traces the study of planning in L2 writing in each of these domains by reviewing key L1 and L2 writing research over the last 30-plus years and by highlighting each study's findings. Prior to presenting the timeline, the following sections provide backgrounds in each of the domains noted above and situate planning within those domains.