Cardiorespiratory fitness (VO2peak) is a clinical endpoint of immense importance following a diagnosis of breast cancer (BC). VO2peak is a strong predictor of overall quality of life, fatigue and other patient-reported outcomes (PROs) among women with early-stage BC. Furthermore, VO2peak is also strongly correlated with biomarkers of BC prognosis and cardiovascular disease. Finally, VO2peak and other measures of cardiorespiratory fitness are well-established predictors of cardiovascular and all-cause mortality in noncancer populations while regular exercise (a major determinant of VO2peak) is associated with a 50% reduction in the risk of mortality, relative to inactive women, following a BC diagnosis. Clearly, interventions that optimally improve VO2peak stand to have considerable clinical benefit for women with BC. Moderate-intensity aerobic training is associated with statistically significant but modest improvements in VO2peak among women with early BC. Based on this data, the American Cancer Society and Institute of Medicine issued guidelines recommending that all cancer patients should exercise e5 days/week for e30 minutes at a moderate intensity. However, no study to date has investigated the level and format of aerobic training that produces optimal improvements in VO2peak and other endpoints in any cancer population. We propose a prospective, three-arm, randomized trial to compare the effects of two aerobic training interventions relative to an attention-control group among early-stage BC patients who have completed primary cancer therapy. This grant will have the following primary and secondary aims: Primary aim: To compare the effect of high-intensity training to moderate-intensity training, relative to attention-control, on VO2peak. Secondary aims: (1) to examine the effects on physiological mediators of VO2peak, (2) To compare the effects on PROs, (3) to compare the effects on biomarkers of BC recurrence and mortality (i.e., metabolic homeostasis and systemic inflammation). This study will address several fundamental but, currently unanswered questions regarding the role of exercise training in the management of breast cancer. Information gained from this clinical trial will inform the design in future-planned studies further investigating the role of exercise training across the breast cancer survivorship continuum. Finally, the mechanistic findings will provide insight into how to refine exercise training interventions to maximize improvements in VO2peak and associated outcomes.