OBJECTIVE: This study identified barriers and facilitators to the high-fidelity implementation of assertive community treatment. METHODS: As part of a multistate implementation project for evidence-based practices, training and consultation were provided to 13 newly implemented assertive community treatment teams in two states. Model fidelity was assessed at baseline and at six, 12, 18, and 24 months. Key informant interviews, surveys, and monthly on-site visits were used to monitor implementation processes related to barriers and facilitators. RESULTS: Licensing processes of the state mental health authority provided critical structural supports for implementation. These supports included a dedicated Medicaid billing structure, start-up funds, ongoing fidelity monitoring, training in the model, and technical assistance. Higher-fidelity sites had effective administrative and program leadership, low staff turnover, sound personnel practices, and skilled staff, and they allocated sufficient resources in terms of staffing, office space, and cars. Lower-fidelity sites were associated with insufficient resources, prioritization of fiscal concerns in implementation, lack of change culture, poor morale, conflict among staff, and high staff turnover. In cross-state comparisons, the specific nature of fiscal policies, licensing processes, and technical assistance appeared to influence implementation. CONCLUSIONS: State mental health authorities can play a critical role in assertive community treatment implementation but should carefully design billing mechanisms, promote technical assistance centers, link program requirements to fidelity models, and limit bureaucratic requirements. Successful implementation at the organizational level requires committed leadership, allocation of sufficient resources, and careful hiring procedures.