Project Summary/Abstract Breast cancer is the most diagnosed cancer in women in the United States, and the population of breast cancer survivors (BCS) is growing. Cancer-related fatigue (CRF) is the most common long term side effect of breast cancer disease and treatment. CRF is associated with poorer quality of life and decreased physical and psychological well-being. Therefore, identifying strategies to manage CRF for breast cancer survivorship is critical. Strong evidence indicates increased physical activity participation is associated with lower CRF levels in BCS and effects are superior to pharmacologic intervention. Therefore, physical activity is recommended as the first-line treatment for CRF. However, most BCS do not meet physical activity guidelines and elevated CRF is associated with lower physical activity levels in BCS. Mobile health (mHealth) interventions may be a promising intervention delivery tool for BCS with CRF because of their increased convenience, flexibility, and scalability potential, and decreased burden. However, no existing mHealth physical activity interventions have specifically targeted BCS with CRF. The purpose of this study is to use the preparation phase of the Multiphase Optimization Strategy (MOST) to explore the effects of different physical activity promotion intervention components in BCS with CRF, and better understand the multi-level intervention component preferences for this population to inform future intervention development. This project will include secondary quantitative data analyses from the Fit2Thrive study, an mHealth physical activity intervention for BCS that used the optimization phase of MOST. Analyses will explore whether feasibility, acceptability, fidelity, and physical activity outcomes varied in the subgroup of BCS participants with elevated CRF. Next, a needs assessment (questionnaires and semi- structured interviews) will assess multi-level stakeholders’ (BCS with fatigue, experts, and community partners) perceived barriers and facilitators of mHealth physical activity intervention uptake in BCS with CRF. This fellowship offers a significant training opportunity in mHealth behavioral intervention design and dissemination, and quantitative and qualitative data analysis. The proposed aims will create an evidence-based foundation for the design and development of future mHealth physical activity interventions for BCS with CRF to promote health and well-being throughout survivorship.