The Institute of Education Sciences (IES) and the What Works Clearinghouse (WWC) have identified topic areas that present a wide range of this nation's most pressing issues in education (e.g., middle school math, beginning reading, and character education). Within each selected topic area, the WWC collects studies of interventions (i.e., programs, products, practices, and policies) that are potentially relevant to the topic area through comprehensive and systematic literature searches. The studies collected are then subjected to a three-stage review process. This paper presents the revised WWC evidence standards for reviewing studies. This publication contains the following sections: (1) What Works Clearinghouse Study Design Classification; (2) What Works Clearinghouse Extent of Evidence Categorization; (3) What Works Clearinghouse Improvement Index; (4) What Works Clearinghouse Intervention Rating Scheme; (5) Technical Details of WWC-Conducted Computations; (6) What Works Clearinghouse Tutorial on Mismatch between Unit of Assignment and Unit of Analysis; and (7) Teacher-Intervention Confound. Individual sections contain footnotes, references, tables, bibliographies, and appendices.