Research shows that summer literacy learning for disadvantaged students tends to be markedly slower than that for more advantaged students. Indeed, these out-of-school summer learning differences may be the key cause of the widening achievement gaps between students of lower and higher socioeconomic levels. As a possible mechanism by which schools may close achievement gaps, summer school is of considerable policy importance. Yet, educators have developed few high-quality summer programs and even fewer have conducted rigorous studies of their potential impacts. This study investigated the effectiveness of a replicable program, called KindergARTen Camp, in preventing the summer achievement slide for 94 public-school students from high-poverty schools in Baltimore. By contrasting outcomes for these students to those of 102 similar control students who did not experience the program, our study generates interpretable causal evidence of the effects of the KindergARTen Camp on achievement. Positive and statistically significant effects for the Letter Naming and Dictation outcomes indicated that the reading achievement gains of KindergARTen Camp students were greater than those for the control students. For Letter Naming, the average treatment student outpaced the average control student by 8 percentile points, or moved past 8% of the control students on the posttest. For the Dictation outcome, the average KindergARTen student outperformed the average control child by more than 6 percentile points. In addition, better attendance was related to strong improvements in students' posttest outcomes with a 5 to 6 percentile point improvement on the outcomes associated with each 10% increase in attendance. These findings suggest that KindergARTen Camp made differences of both statistical and practical significance for participating students. (author abstract)