Abstract The goal of this study is to test the effectiveness and describe implementation of a home based exercise intervention for persons with an implantable cardioverter defibrillator (ICD). This study is based on the NIH stage model of behavioral interventions (stage III effectiveness) that aligns with the strategic mission of the NHLBI, to optimize clinical and implementation research to improve health and reduce disease (Obj #6). The study intervention is based on the protocol and monitoring components that were developed in prior RCTs and a pilot study conducted by our team. From these studies, we created a home based exercise program called Exercise-ICD (E-ICD) for testing in a broader range of ICD patients, that assists individuals to start and monitor exercise safely after an ICD. An randomized two group (E-ICD vs. usual care) pragmatic effectiveness mixed- methods study is proposed, to determine the effectiveness of the E-ICD intervention and describe implementation by cardiac rehabilitation (CR) clinical staff. Two hundred ten (N=210) patients in 3 local study sites (70/site) in the greater Seattle, WA area will participate. E-ICD is guided by the Reach-Effectiveness- Adoption-Implementation-Maintenance (RE-AIM) model. The E-ICD intervention consists of 12 weeks of home walking exercise using exercise prescriptions and protocols validated in our previous work. The primary effectiveness outcome is patient physical activity (steps/day) after the E-ICD intervention at 3 months. We will determine the number of patients who reach the public health walking goal of moderate level exercise of 150 minutes/week at the end of the study. Measures will be taken at baseline, after the intervention at 3 months, and at 6 and 12 months to determine maintenance. The specific aims are to: 1) Test the hypothesis that participants randomized to E-ICD vs. usual care (UC) will demonstrate increased physical activity (minutes walked/week) [primary outcome], health related quality of life, gait speed, mobility, exercise self-efficacy; and reduced ICD shock anxiety and depression at 3 months (E-ICD Effectiveness), 2) Assess longer term effects of E-ICD on participant and health care system Maintenance, captured by 1) participant retention, satisfaction, and adherence at 12 months (total minutes/week), and 2) clinician and institutional intent to sustain use of the E-ICD intervention after the study, and 3) Evaluate the reach, adoption, and implementation of E-ICD by CR clinical staff. Reach will be assessed by the diverse patient participation rate, representativeness, and reasons for non-interest. Adoption is the participation rate and representativeness of the settings and staff who participate, and factors related to adoption. Implementation will be assessed by delivery fidelity of E-ICD and workflow adaptations, implementation costs, and consistency of delivery across settings. Intervention effectiveness will be described by participant social determinants ...