This research brief reviews research on why and how family involvement matters for elementary school children's learning and socio-emotional development. Specifically, it summarizes the latest evidence base on effective involvement for elementary school children--that is, the studies that link family involvement in elementary schools to children's outcomes. It also profiles programs that have been evaluated to show what works to promote family involvement in the elementary school years. The brief concludes with implications for policy, practice, and research. This review underscores the importance of considering these three family involvement processes as policymakers, practitioners, and researchers endeavor to create systematic, developmental, and comprehensive programs for family involvement. With family involvement processes in place during the elementary school years, children will be poised for smooth transitions to middle and high school and for success in these even more complex educational settings. A list of family involvement resources; and a list of complementary learning resources from the Harvard Family Research Project are also included. Method is appended. (Contains 1 figure and 61 notes.)