"Attrition" is the loss of sample during the course of a study. It occurs when individuals initially randomly assigned in a study are not included when researchers examine the outcome of interest. Attrition is a common issue in education research, and it occurs for many reasons. The What Works Clearinghouse (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 attrition?; (2) Why does attrition matter?; and (3) How does the WWC handle attrition? [For more information about the attrition 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.]