As a systemic approach to improving educational practice through research, ‘What Works’ has come under repeated challenge from alternative approaches, most recently that of improvement science. While ‘What Works’ remains a dominant paradigm for centralized knowledge-building efforts, there is need to understand why this alternative has gained support, and what it can contribute. I set out how the core elements of experimental and improvement science can be combined into a strategy to raise educational achievement with the support of evidence from randomized experiments. Central to this combined effort is a focus on identifying and testing mechanisms for improving teaching and learning, as applications of principles from the learning sciences. This article builds on current efforts to strengthen approaches to evidence-based practice and policy in a range of international contexts. It provides a foundation for those who aim to avoid another paradigm war and to accelerate international discussions on the design of systemic education research infrastructure and funding.