Research Findings: Multiple student cohorts were longitudinally tracked and student participation in a summer program bridging the 1st- and 2nd-grade academic years was recorded to examine selection and efficacy issues related to a summer school implementation in the Pacific Northwest. The estimation of regression discontinuity models uncovered evidence of a local average treatment effect. At the cutscore for program admission, participating students had estimated summer oral reading fluency gains approximately 0.40 SD larger than those of nonparticipants. Further examination of the literacy outcomes among the sample of cutscore eligible students revealed that struggling readers who participated in the summer program increased their level of reading fluency relative to struggling readers who declined an invitation to participate. However, the advantage gained by cutscore eligible participants was not sustained over the subsequent academic year. Practice or Policy: These results suggest that supplemental summer instruction delivered to at-risk students may promote literacy gains during the otherwise challenging summer months and thereby serve as a useful intermediary tool for K-12 stakeholders seeking to keep struggling readers on track toward proficiency. (Contains 3 figures, 4 tables, and 4 footnotes.)