We propose to add a direct measure of sedentary behavior to The Alberta Moving Beyond Breast Cancer (AMBER) Cohort Study. The AMBER Cohort was funded by the Canadian Institutes of Health Research. First, we propose to administer the activPAL devices to 1,500 newly diagnosed female non-metastatic breast cancer survivors enrolling in the AMBER Cohort (Objective 1). The device will be worn on the thigh for 7 days at study baseline and at one- and three-years post-diagnosis. Participants undergoing surgery will complete the baseline measures after they have been cleared to return to normal activities, allowing up to 4 to 8 weeks of recovery prior to completion of the baseline measurements for most women. Using these data, and information available from the parent cohort, we will investigate: the cross-sectional relation between activPAL measured sedentary behavior and cardiorespiratory fitness at baseline (Objective 2); changes in activPAL measured sedentary time from study baseline to 1 and 3 years post-diagnosis (Objective 3), and, the prospective relation between activPAL measured sedentary behavior at baseline and cardiorespiratory fitness measured at one year (Objective 4). In all analyses we will employ analytic techniques (i.e., statistical models, stratified analysis) that will identify and/or adjust for important confounding factors, such as diagnosis and treatment factors (e.g., stage, surgery type, type and timing of adjuvant treatments), psychosocial factors (e.g., depression, fatigue), moderate-vigorous exercise, and other critical covariates (e.g., age, comorbidities).