Despite decades of policy intervention to increase college entry among low-income students, substantial inequalities in college going by family income remain. Policy makers have largely overlooked the summer after high school as an important time period in students' transition to college. During the post-high school summer, however, students must complete a range of financial and informational tasks prior to college enrollment, yet no longer have access to high school counselors and have not engaged yet with their college community. Moreover, many come from families with little college-going experience. Recent research documents summer attrition rates ranging from 10 to 40 percent among students who had been accepted to college and declared an intention to enroll in college as of high school graduation. Encouragingly, several experimental interventions demonstrate that students' postsecondary plans are quite responsive to additional outreach during the summer months. Questions nonetheless remain about how to maximize the impact and cost effectiveness of summer support. This chapter reports on several randomized trials to investigate the impact of summer counselor outreach and support as well as the potential roles for technology and peer mentoring in mitigating summer attrition and helping students enroll and succeed in college. The authors conclude with implications for policy and practice.