Obesity is a growing health problem in the United States and, increasingly, around the world. Excess body weight has been linked to an increased risk of postmenopausal breast cancer, and growing evidence also suggests that obesity is associated with poor prognosis in women diagnosed with early-stage breast cancer. Dozens of studies demonstrate that women who are overweight or obese at the time of breast cancer diagnosis are at increased risk of cancer recurrence and death compared with leaner women, and some evidence suggests that women who gain weight after breast cancer diagnosis may also be at increased risk of poor outcomes. In this review, we describe the evidence linking obesity to breast cancer recurrence, discuss the potential biological mechanisms through which weight could impact breast cancer prognosis, and review the weight-loss intervention studies that have been performed in breast cancer populations to date.