In June of 2004, a national policy forum was held, sponsored by the U.S. Department of Justice and organized by the Council for Excellence in Government and the Coalition for Evidence Based-Policy to address whether and how rigorous evidence about what actually works can be the key to making substantial progress against crime and substance abuse. The forum consisted of two primary panel presentations with follow up roundtable question and answer discussions. The first panel presented discussed rigorous evidence as the engine of progress in medicine and welfare policy and the implications for crime and substance abuse policy. The second panel presented discussed what works in crime and substance abuse policy and presented examples of interventions found highly effective in randomized controlled trials. The report concludes with recommendations for consideration in the development and effective use of rigorous evidence on what works in crime and substance abuse policy. The recommendations are designed to be implemented and consist of: (1) Federal agencies developing a concise, uniform, user-friendly set of principles on what constitutes “rigorous evidence” of an intervention’s effectiveness; (2) agencies launching a major strategy to build the knowledge base of evidence-backed crime/substance-abuse interventions; (3) each agency focusing its discretionary funds for research/evaluation to the maximum extent on the above strategy to build the knowledge base of evidence-backed interventions; (4) each agency establishing or contributing to a “what works” Web site providing information to practitioners on evidence-backed intervention; (5) agency crime/substance abuse grant programs requiring applicants to provide a concrete strategy for implementation of evidence-backed interventions with fidelity; and (6) each agency undertaking a major effort to educate the policy and grantee communities on the value of evidence-based reforms and provide technical assistance to facilitate implementation.