Schools and districts are adopting out-of-school-time (OST) programs such as after-school programs and summer schools to supplement the education of low-achieving students. However, research has painted a mixed picture of their effectiveness. To clarify OST impacts, this synthesis examined research on OST programs for assisting at-risk students in reading and/or mathematics. Researchers analyzed 35 OST studies that employed control or comparison groups and met other inclusion criteria. Meta-analyses indicated small but statistically significant positive effects of OST on both reading and mathematics student achievement and larger positive effect sizes for programs with specific characteristics such as tutoring in reading. Whether the OST program took place after school or during the summer did not make a difference in effectiveness.