Attaining and demonstrating treatment fidelity is critical in the development and testing of evidence-based interventions. Treatment fidelity refers to the extent to which an intervention was implemented in clinical testing as it was conceptualized and is clearly differentiable from control or standard-of-care interventions. In clinical research, treatment fidelity is typically attained through intensive training and supervision techniques and demonstrated by measuring therapist adherence and competence to the protocol using external raters. However, in occupational and physical therapy outcomes research, treatment fidelity methods have not been used, which, in our view, is a serious gap that impedes novel treatment development and testing in these rehabilitation fields. In this article, we describe the development of methods to train and supervise therapists to attain adequate treatment fidelity in a treatment development project involving a novel occupational and physical therapy-based intervention. We also present a data-driven model for demonstrating therapist adherence and competence in the new treatment and its differentiation from standard of care. In doing so, we provide an approach that rehabilitation researchers can use to address treatment fidelity in occupational and physical therapy-based interventions. We recommend that all treatment researchers in rehabilitation disciplines use these or similar methods as a vital step in the development and testing of evidence-based rehabilitation interventions.