The main purpose of this article is to review studies that have investigated the effects of three types of planning (rehearsal, pre-task planning, and within-task planning) on the fluency, complexity, and accuracy of L2 performance. All three types of planning have been shown to have a beneficial effect on fluency but the results for complexity and accuracy are more mixed, reflecting both the type of planning and also the mediating role of various factors, including task design and implementation variables and individual difference factors. A secondary purpose is to outline a theory that can account for the role that planning plays in L2 performance. The article concludes with a list of limitations in the research to date.