From 2013 to 2016, the Weikart Center for Youth Program Quality and the National Summer Learning Association (NSLA) partnered to design and test the Summer Learning Program Quality Intervention (SLPQI). The Summer Learning Program Quality Assessment, which defines quality as the presence of identified practices that benefit young people, was created, piloted, and tested as part of the study. The study involved hundreds of observational assessments from 216 sites and additional data collected via surveys, focus groups, interviews, and student assessments. Feedback on the tool and/or the process was provided by participating staff, assessors, networks, and intermediaries. The SLPQI study demonstrated that a summer program quality improvement process is manageable. It showed that differences in performance between high- and low-quality sites are measurable in low-stakes accountability contexts, allowing extra supports and resources to be aimed where they are most needed. When given performance feedback along with extra training and coaching, staff are able to improve the quality of initially lower-performing sites. Most of all, the SLPQI illustrated that quality programming and quality practices matter. Young people in summer programs characterized by responsive, high-quality practices made greater academic progress.