ow does a school system with poor performance become good? And how does one with good performance become excellent? These were the questions policymakers and education leaders asked us in the wake of our 2007 report How the world’s best performing school systems come out on top, in which we examined the common attributes of high-performing school systems. In this new report, How the world’s most improved school systems keep getting better, we attempt to answer these questions. We analyzed 20 systems from around the world, all with improving but differing levels of performance, examining how each has achieved significant, sustained, and widespread gains in student outcomes, as measured by international and national assessments. Based on over 200 interviews with system stakeholders and analysis of some 600 interventions carried out by these systems—together comprising what we believe is the most comprehensive database of global school system reform ever assembled—this report identifies the reform elements that are replicable for school systems elsewhere as they move from poor to fair to good to great to excellent performance. The systems we studied were: Armenia, Aspire (a US charter school system), Boston (Massachusetts), Chile, England, Ghana, Hong Kong, Jordan, Latvia, Lithuania, Long Beach (California), Madhya Pradesh (India), Minas Gerais (Brazil), Ontario (Canada), Poland, Saxony (Germany), Singapore, Slovenia, South Korea, and Western Cape (South Africa).