Although advances in prevention science over the past two decades have produced a growing list of tested and effective programs and policies for preventing adolescent delinquency and drug use, widespread dissemination and high-quality implementation of effective programs and policies in communities has not been achieved. The Community Youth Development Study (CYDS) is a randomized, community-level trial of the Communities That Care (CTC) system for promoting science-based prevention in communities. This study compares 12 community prevention coalitions implementing the CTC system in 12 intervention communities as part of the CYDS to prevention coalitions located in the 12 control communities. As hypothesized, the CYDS coalitions implemented significantly more of the CTC core intervention elements, and also implemented significantly greater numbers of tested, effective prevention programs than the prevention coalitions in the control communities. Implications of the findings for efforts to achieve widespread dissemination of effective prevention programs, policies, and practices are discussed. (Contains 2 figures and 1 table.)