This chapter summarizes research on the timing of written and oral corrective feedback (CF) on L2 grammatical errors. A careful analysis of a comprehensive array of CF timing studies indicates that there are learning benefits from both immediate and delayed CF. The analysis also demonstrates the need for more uniform construct definitions and more rigidly designed studies in the area of CF timing research. The chapter then explains several theoretical proposals about how immediate and delayed CF facilitate L2 development. These proposed explanations include sociocultural theory, Transfer Appropriate Processing, skill acquisition theory, and cognitive comparison via reactivation and reconsolidation. In addition, this chapter discusses the potential implications that research and theory might have for L2 pedagogy, concluding that at this juncture, the literature provides teachers with justifications for using both immediate and delayed CF. Finally, the chapter ends with recommendations about what future research might best be conducted in the area of CF timing.