Abstract Mounting evidence suggests that obesity at breast cancer diagnosis is associated with poorer survival. Far fewer studies have described weight patterns in the post-diagnosis period or explored how post-diagnosis weight patterns impact cancer outcomes. Those that have, had important limitations making it challenging to draw strong conclusions and direct resources to the most appropriate populations. However, the adverse role of obesity in breast cancer risk has driven the development of diet and exercise recommendations and lifestyle behavioral programs for cancer prevention and control. Studies have shown adherence to these recommendations can improve overall quality of life, body composition, and surrogate markers of mortality in women with breast cancer. Unfortunately, few breast cancer survivors currently meet the nutrition and physical activity guidelines or report discussing diet and exercise with their oncologist. This highlights an important evidence-practice gap which partially exists because research to date has focused on the efficacy of lifestyle interventions with less attention on how these interventions might be translated into clinical practice. Guided by the RE-AIM (Reach, Effectiveness, Adoption, Implementation and Maintenance) framework, the goal of this application is to clarify the potential benefits of bringing behavioral lifestyle programming into routine oncology care and demonstrate how to do so in a scalable, sustainable, and cost-effective way using breast cancer as a model. First, we will describe weight patterns after breast cancer diagnosis by leveraging electronic health record data from a large, diverse healthcare network. We will apply novel longitudinal methods to define the population who could benefit from lifestyle counseling and to inform the most appropriate time to intervene. Second, we will conduct a mixed methods study to explore the experiences of women with breast cancer participating in a nutrition and exercise intervention during chemotherapy. From the patients' perspective, we aim to uncover motivations, preferences, and perceived barriers to and facilitators of adhering and sustaining a nutrition and exercise intervention during active cancer treatment. Lastly, we will estimate the cost-effectiveness of the same comprehensive nutrition and exercise intervention for breast cancer patients undergoing chemotherapy to provide critical data that can help inform the scaling-up of behavioral interventions in the oncology setting. An interdisciplinary team of renowned investigators was assembled from the Yale School of Public Health and the Yale School of Medicine to provide direct research support and professional development mentorship to the applicant. The proposed research and the collaborative training environment at Yale will enable the applicant to hone her existing skillsets and gain specialized expertise that will position her well for a competitive post-doctoral position and a long-term...