This article describes the degree to which high fidelity implementation of the Communities That Care (CTC) prevention operating system was reached during the first 18 months of intervention in 12 communities in the Community Youth Development Study, a 5-year group randomized controlled trial designed to test the efficacy of the CTC system. CTC installation in these communities included the delivery of six CTC trainings from certified CTC trainers at each site, the active involvement of locally selected and community-based CTC community coordinators, ongoing monitoring of progress using the CTC milestones and benchmarks, and proactive technical assistance and coaching. CTC implementation fidelity ratings averaged across three groups of raters show that between 89% and 100% of the CTC milestones in the first four phases of CTC implementation were "completely met" or "majority met" in the 12 intervention communities, indicating that the first four phases of the CTC system have been well implemented in the communities in this trial. (Contains 4 tables.)