There is a growing belief that access to academic opportunities during the summer can help close the achievement gap between low-income students and their higher-income peers. But while significant research is emerging on summer programs for school-age children, information on the preschool period is limited. The Expanding Children's Early Learning (ExCEL) Network is examining the potential benefits of a summer enrichment program to promote school readiness skills for children about to enter kindergarten. The ExCEL Summer Project initiated a learning network of practitioners, policymakers, and researchers with the aim of connecting theories about the promise of summer programs preceding kindergarten, existing research about features of effective summer programs for school-age children, and the day-to-day realities of implementing these programs for rising kindergarten students. This brief introduces the ExCEL Summer Project and summarizes lessons learned about three implementation issues -- recruitment, attendance, and family involvement -- that affect all summer programs but may pose unique challenges and opportunities for programs for rising kindergartners. (author abstract)