This "What Works Clearinghouse Procedures and Standards Handbook (Version 3.0)" provides a detailed description of the standards and procedures of the What Works Clearinghouse (WWC). The remaining chapters of this Handbook are organized to take the reader through the basic steps that the WWC uses to develop a review protocol, identify the relevant literature, assess research quality, and summarize evidence of effectiveness. Organizational procedures used by the WWC to ensure an independent, systematic, and objective review are described in the appendices. Table I.1 provides a summary of the remaining chapters and associated appendices. The main differences between this version of the procedures and standards and the previous version (Version 2.1) are in clarity, detail, and scope. The organization of the Handbook, as well as all text, was reviewed and modified to support clarity; additionally, examples have been added throughout. There is more detail on the specific procedures and standards used by the WWC, including how to deal with missing data, random assignment probabilities, and cluster-level designs. Finally, whereas the previous version focused almost exclusively on intervention reports, this version provides information on other key WWC products, which include practice guides, single study reviews, and quick reviews. As the WWC continues to refine processes, develop new standards, and create new products, the "What Works Clearinghouse Procedures and Standards Handbook" will be revised to reflect these changes. Appended are: (1) Staffing, Reviewer Certification, and Quality Assurance; (2) Policies for Searching and Prioritizing Studies for Review; (3) The WWC Study Review Process; (4) Pilot Regression Discontinuity Design Standards; (5) Pilot Single-Case Design Standards; (6) Magnitude of Findings for Randomized Controlled Trials and Quasi-Experimental Designs; and (7) Statistical Significance for Randomized Controlled Trials and Quasi Experimental Designs. [See ED503772 to view previous version of this guide.]