Research shows that during summer, low-income and non-white students fall behind academically compared with their more-affluent and white peers. To understand whether and how district-led voluntary summer learning programs can improve outcomes for low-income students, The Wallace Foundation initiated the National Summer Learning Project in 2011 in five urban districts. The RAND Corporation's six-year study of the National Summer Learning Project culminates in this final report about districts' implementation of their summer learning programs. This second edition updates guidance first published in 2013 and is intended for district leaders and their partners across the country who are interested in launching or improving summer learning programs. Based on thousands of hours of observations, interviews, and surveys, it presents the best available guidance about how to establish and sustain effective programs. The most emphatic recommendation is to commit in the fall to a summer program, and start active planning by January with a program director who has at least half of his or her time devoted to the job. Other recommended practices include recruiting teachers with content knowledge, scheduling the program to include at least 25 hours of math and 34 hours of language arts, adopting recruitment and attendance policies aimed at high attendance rates, using written curricula that align with school-year standards, keeping a high level of engagement between adults and students even during transitions and time outside of class, and designing the program to consider cost-saving measures.