The What Works Clearinghouse (WWC) uses the term "baseline equivalence" when determining if the intervention group (those that received the intervention of interest) and the comparison group (those that did not receive the intervention) had characteristics that were similar enough ("equivalent") at the start of the study (at "baseline"). 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, policies, and practices, which the WWC calls "interventions." WWC Standards Briefs explain the rules the WWC uses to assess the quality of studies. This brief discusses: (1) What is baseline equivalence?; (2) Why does baseline equivalence matter?; and (3) How does the WWC determine baseline equivalence? [For more information about the baseline equivalence standard and other WWC standards, see "Standards Handbook. Version 4.0. What Works Clearinghouse™," ED577036 and "Procedures Handbook. Version 4.0. What Works Clearinghouse™," ED577035.]