The new federal education law, the Every Student Succeeds Act (ESSA), envisions a powerful role for states in managing the evidence base behind school improvement efforts. Not only must they certify that interventions meet the "evidence-based" requirements spelled out in the law, they also must monitor and evaluate federally funded school-improvement efforts going forward. There is only one problem: states have never played such a role before. In order to fulfill this obligation, states will need a scalable model of impact evaluation which could operate at the local level, where decisions are being made. In this essay, the author describes how a state could use the authority and resources provided by the ESSA to launch a system of "efficacy networks," or collections of local agencies committed to measuring the impact of the interventions they are using. An overlapping system of efficacy networks working with local agencies would create a mechanism for continuous testing and improvement in U.S. education. More than any single policy initiative or program, such a system would be a worthwhile legacy for any state leader.