: Treatment fidelity pertains to the methodological strategies used to monitor and enhance the reliability and validity of behavioral interventions. Approaches to establishing treatment fidelity in Web-based interventions differ from those used in interventions that are delivered in person. : The aim of this study was to describe a methodology for ensuring treatment fidelity in a Web-based cognitive behavioral stress management intervention. : The intervention Coping With Cancer Workbook, adapted for Web-based delivery from an in-person intervention, was tested in a randomized controlled trial with 123 breast cancer survivors. Strategies for ensuring treatment fidelity were implemented and assessed. : The National Institutes of Health Behavior Change Consortium Treatment Fidelity Guidelines were relevant to establishing treatment fidelity for the Web-based intervention. : Web-based delivery of behavioral interventions is both a strength and a threat to treatment fidelity. Investigators must be cognizant of the elements of treatment fidelity and implement strategies to monitor and improve the reliability and validity of Web-based interventions.