The What Works Clearinghouse (WWC) at the Institute of Education Sciences reviews rigorous research on educational practices, policies, programs, and products with a goal of identifying 'what works' and making that information accessible to the public. One critique of the WWC is the need to more closely examine 'what works' for whom, in which settings, and for which outcomes. The purpose of this article is to create evidence maps of the WWC study data to display the student populations, settings, and outcomes included in the evidence base. Findings show that the WWC study data include a broad range of students, settings, and outcomes relevant to education in the U.S., yet missing data on sample characteristics are a concern. The WWC evidence base also includes relatively few studies focused on specific student populations or some outcome domains. Finally, findings suggest that 'what worked', or the extent to which studies found favorable results, meaningfully depended on which outcomes were examined in the studies. The findings have implications for researchers, funders of research, and the WWC. [This paper will be published in the "Journal of Research on Educational Effectiveness."]