The purpose of this Guide is to advise researchers, policymakers, and others on when it is possible to conduct a high-quality randomized controlled trial in education at reduced cost. Well-designed randomized controlled trials are recognized as the gold standard for evaluating the effectiveness of an intervention (i.e., program or practice) in many diverse fields, such as medicine, welfare and employment, psychology, and education. But they have been relatively rare in education, in part because of a perception among policymakers, researchers, and others that such studies are too costly and too administratively burdensome on schools to be practical. This guide explains that, in many circumstances, it may in fact be possible to conduct a randomized controlled trial at modest cost and with minimal burden, by measuring outcomes using school-administered test scores or other administrative data that are already collected for other purposes. This guide includes two main sections: (1) Conditions that offer the opportunity to conduct a randomized controlled trial at reduced cost; and (2) Examples of well-designed randomized controlled trials conducted at reduced cost.