This paper describes the development, application, and results of an implementation monitoring component of the Communities That Care (CTC) prevention framework used in the Community Youth Development Study (CYDS) to ensure high-fidelity prevention program implementation. This system was created based on research that community-based implementation of evidence-based prevention programs often includes adaptations in program design, content, or manner of delivery (Gottfredson and Gottfredson, Journal of research in crime and delinquency, 39, 3-35, 2002; Hallfors and Godette, Health Education Research, 17, 461-470, 2002; Wandersman and Florin, American Psychologist, 58, 441-448, 2003). A lack of fidelity to the implementation standards delineated by program designers is one indicator of a gap between prevention science and practice which can lessen the likelihood that communities will realize the positive participant effects demonstrated in research trials. By using the CTC model to select and monitor the quality of prevention activities, the 12 CYDS communities replicated 13 prevention programs with high rates of adherence to the programs' core components and in accordance with dosage requirements regarding the number, length, and frequency of sessions. This success indicates the potential of the CTC program implementation monitoring system to enhance community Prevention Delivery Systems (Wandersman et al. American Journal of Community Psychology, this issue) and improve the likelihood of desired participant changes.