Written corrective feedback has been subject to increasing attention in recent years, in part because of the conceptual controversy surrounding it and in part because of its ubiquitous practice. This study takes a meta-analytic approach to synthesizing extant empirical research, including 21 primary studies. Guiding the analysis are two questions: Does written corrective feedback help to improve the grammatical accuracy of second language writing? What factors might mitigate its efficacy? Results show that written corrective feedback can lead to greater grammatical accuracy in second language writing, yet its efficacy is mediated by a host of variables, including learners' proficiency, the setting, and the genre of the writing task.