A diagnosis of breast cancer is associated with treatments that have physiologic effects beyond the intended curative therapy. The first section of this chapter provides and integrative physiology review of the effects of breast cancer treatment on the body systems used by and affected by physical activity, including effects of chemotherapy, radiation, and surgery. In later sections, we review the literature on physical activity and breast cancer from the point of diagnosis and for the balance of life. The efficacy of physical activity for supportive cancer care outcomes is reviewed separately from the purported usefulness of physical activity for disease-free and overall survival from breast cancer. The current evidence supports the safety of physical activity during and after breast cancer therapy. The supportive cancer care outcomes for which there is sufficient evidence of efficacy during and after breast cancer treatment include fitness, fatigue, body size, and quality of life. Further, there is growing evidence that upper body exercise does not pose additional risk for negative lymphedema outcomes among survivors with and at risk for lymphedema. For overall survival, the evidence is largely observational, with sufficient evidence that physical activity does confer benefit. Finally, we outline future directions for research on physical activity among breast cancer survivors, including expanding to focus on subsets of the population not included in most prior studies (minority women and older women), tailoring of interventions to stages of cancer most likely to benefit, expanding to study women with metastatic cancer, and new modes of exercise, such as team sports, martial arts, and Pilates.