To help inform drug abuse prevention research in school settings about the issues surrounding implementation, we conducted a review of the fidelity of implementation research literature spanning a 25-year period. Fidelity has been measured in five ways: (1) adherence, (2) dose, (3) quality of program delivery, (4) participant responsiveness and (5) program differentiation. Definitions and measures of fidelity were found not to be consistent across studies, and new definitions are proposed. While there has been limited research on fidelity of implementation in the social sciences, research in drug abuse prevention provides evidence that poor implementation is likely to result in a loss of program effectiveness. Studies indicate that most teachers do not cover everything in a curriculum, they are likely to teach less over time and training alone is not sufficient to ensure fidelity of implementation. Key elements of high fidelity include teacher training, program characteristics, teacher characteristics and organizational characteristics. The review concludes with a discussion of the tension between fidelity and reinvention/adaptation, and ways of resolving this tension. Recommendations are made for developing a consistent methodology for measuring and analyzing fidelity of implementation. Further, researchers and providers should collaborate to develop ways of introducing flexibility into prevention programs.