We present results from a meta-analysis of 37 experimental and quasi-experimental studies of summer programs in mathematics for children in Grades pre-K-12, examining what resources and characteristics predict stronger student achievement. Children who participated in summer programs that included mathematics activities experienced significantly better mathematics achievement outcomes, compared to their control group counterparts. We find an average weighted impact estimate of +0.10 standard deviations on mathematics achievement outcomes. We find similar effects for programs conducted in higher- and lower-poverty settings. We undertook a secondary analysis exploring the effect of summer programs on non-cognitive outcomes and found positive mean impacts. The results indicate that summer programs are a promising tool to strengthen children's mathematical proficiency outside of school time.