Exercise training is a safe and efficacious intervention for breast cancer patients following the completion of primary therapy (i.e., chemotherapy and radiation). In contrast, the role of exercise during primary BC therapy is less well accepted. Indeed, a perception exists that the conduct of scientifically rigorous exercise trials may not be possible during therapy due to poor adherence and attrition rates, elevated risk of adverse events, and significant patient selection bias. Nevertheless, a growing number of studies indicate that exercise is an effective therapy to mitigate certain cancer-related toxicities (e.g., deconditioning, fatigue) with adherence and attrition rates well within conventionally accepted levels, and with minimal AEs. Although the acceptance of exercise during therapy has made significant strides over the past decade, the prevailing dogma is still that exercise should be avoided during primary therapy. We propose a four-arm RCT to compare the effects and safety of supervised aerobic training performed during, after, or during and after (i.e., continuous AT) standard primary therapy, relative to attention-control, in 160 (n=40/group) early-stage BC patients. This grant has the following objectives: Primary aims: (1) To compare the difference in cardiorespiratory fitness between aerobic training and control during therapy versus aerobic training and control after therapy, (2) To compare the difference in oxygen transport organs that govern the aerobic training - fitness relationship, and (3) to compare the difference in patient-reported outcomes, adverse events, adherence, and attrition rates between study groups. A secondary aim is to compare the effect of continuous aerobic training, relative to all other groups, on primary aims 1 - 3.