Districts are struggling to accelerate students' learning, support their health and wellness, and close the achievement gap. Labeled "worst" in 1987, by 2017 Chicago Public Schools was among the nation's most improved school systems. Chicago's story embeds many lessons about system change. Sharon Greenberg and Anthony S. Bryk illustrate how local research evidence helped leaders target a few high-leverage processes and informed their ongoing efforts to improve outcomes. Achieving quality enactment of these processes at scale depended on building capacity, which, in turn, took time.