This paper presents a critical synthesis of research on written corrective feedback (WCF) and its effects on second language (L2) learning over the past four decades. WCF is an essential component of L2 teaching. However, whether WCF helps has been an issue of considerable debate in the literature. While many researchers have stressed its importance, some others have expressed doubt concerning its effectiveness. The controversy over the effectiveness of WCF was heightened when Truscott argued in 1996 that error correction is ineffective and harmful and others who strongly countered his argument that feedback is effective (e.g. Chandler 2003; Ferris 1999). This article provides an in-depth synthesis and analysis of this area of research, examining key issues and findings as well as the various contentions and concerns raised regarding the effects of WCF. The article concludes with implications for future research and with insights about how to move forwards.