OBJECTIVES: To discuss methods of preservation of treatment fidelity in health behavior change trials conducted in public health contexts. METHODS: The treatment fidelity framework provided by the NIH's Behavioral Change Consortium (BCC) (1) includes five domains of treatment fidelity (Study Design, Training, Delivery, Receipt, and Enactment). A measure of treatment fidelity was previously developed and validated using these categories. RESULTS: Strategies for assessment, monitoring, and enhancing treatment fidelity within each of the five treatment fidelity domains are discussed. The previously created measure of treatment fidelity is updated to include additional items on selecting providers, additional confounders, theory testing, and multicultural considerations. CONCLUSIONS: Implementation of a treatment fidelity plan may require extra staff time and costs. However, the economic and scientific costs of lack of attention to treatment fidelity are far greater than the costs of treatment fidelity implementation. Maintaining high levels of treatment fidelity with flexible adaptation according to setting, provider, and patient is the goal for public health trials.