In 2002, Florida adopted a test-based promotion policy in the third grade in an attempt to end social promotion. Similar policies are currently operating in Texas, New York City, and Chicago and affect at least 17 percent of public school students nationwide. Using individual-level data on the universe of public school students in Florida, we analyze the impact of grade retention on student proficiency in reading one and two years after the retention decision. We use an instrumental variable (IV) approach made available by the relatively objective nature of Florida's policy. Our findings suggest that retained students slightly outperformed socially promoted students in reading in the first year after retention, and these gains increased substantially in the second year. Results were robust across two distinct IV comparisons: an across-year approach comparing students who were essentially separated by the year in which they happened to have been born, and a regression discontinuity design.