Many studies of education interventions make claims about impacts on students' outcomes. Some studies have designs that enable readers to make causal inferences about the effects of an intervention but others have designs that do not permit these types of conclusions. To help policymakers, practitioners, and others make sense of study results, the What Works Clearinghouse (WWC) conducts a thorough assessment of the quality of evidence the study provided on the effectiveness of an intervention, called a WWC study review. Certified reviewers, who have been trained to assess the strength of evidence provided by individual studies, conduct the WWC reviews. Reviews serve as the building blocks for WWC products; they are compiled into these products to provide a review of evidence on interventions of interest to practitioners, researchers, and policymakers. The WWC is an initiative of the U.S. Department of Education's Institute of Education Sciences. The WWC evaluates research studies that look at the effectiveness of education programs, products, practices, and policies, which the WWC calls "interventions." WWC Process Briefs explain how the WWC does different aspects of its work. This brief discusses: (1) What is a WWC study review?; (2) How do individual reviewers assess studies?; (3) How is the rating confirmed and documented?; (4) What if a study does not provide all the information a reviewer needs? [For more information about the review process, see "Standards Handbook. Version 4.0. What Works Clearinghouse™," ED577036 and "Procedures Handbook. Version 4.0. What Works Clearinghouse™," ED577035.]