There has long been interest in using summertime to provide supplemental education to students who need it. But are summer programs effective? This review includes 19 randomized studies on the effects of summer intervention programs on reading and mathematics, based on rigorous quality criteria. In reading, there were two types of summer programs: summer school and summer book reading approaches. In mathematics, there was only summer school. The mean effect of summer school programs on reading achievement were positive (mean ES = +0.23), but there were no positive effects, on average, of summer book reading programs (ES=0.00). In mathematics, positive mean effects were also found for summer programs (ES=+0.17). However, the positive-appearing means for summer schools were not statistically significant in a metaregression, and depended on just two reading and one math study with very large impacts. These successful interventions focused on well-defined objectives with intensive teaching.