Treatment fidelity plays an important role in the research team's ability to ensure that a treatment has been implemented as intended and that the treatment has been accurately tested. Developing, implementing, and evaluating a treatment fidelity plan can be challenging. The treatment fidelity workgroup within the Behavior Change Consortium (BCC) developed guidelines to comprehensively evaluate treatment fidelity in behavior change research. The guidelines include evaluation of treatment fidelity with regard to study design, training of interventionists, delivery and receipt of the intervention, and enactment of the intervention in real-life settings. This article describes these guidelines and provides examples from four BCC studies as to how these recommended guidelines for fidelity were considered. Future work needs to focus not only on implementing treatment fidelity plans but also on quantifying the evaluations performed, developing specific criteria for interpretation of the findings, and establishing best practices of treatment fidelity.