One of the most difficult challenges to effective substance abuse prevention is finding the right balance between maintaining the fidelity of a science-based model prevention program and promoting adaptation of that program to reflect the circumstances of the community where it is being implemented. This state-of-the-art review, prepared for the Center for Substance Abuse Prevention (CSAP), surveys 125 published and unpublished studies related to fidelity and adaptation balance, spanning more than 25 years. An extensive list of references guides researchers to the full body of literature surveyed for this review. The report first defines several key terms and then reviews the relevant research in some detail. It then presents several conclusions drawn from the literature review. The fundamental conclusion is that attention to both program fidelity and adaptation during the complex process of program implementation is critical to successful, sustained implementation of science-based substance abuse prevention programs. In addition, this paper proposes an initial set of guidelines for program implementers and also several unresolved issues that require attention from each of the primary audiences for this work. The paper concludes with a consideration of next steps for CSAP and others in order to advance the understanding of program fidelity and adaptation balance in substance abuse prevention. The executive summary is appended. (Contains 121 references.) (GCP)