Breast cancer (BC) is the most common and second deadliest malignancy among American women. Many factors contribute to BC prognosis, but a key modifiable lifestyle factor is body weight. In this review, we update the reader on the association between adiposity and poor BC outcomes. We summarize the findings from studies that show obesity to be a risk factor for BC recurrence and reduced survival, including research that shows that treatment with aromatase inhibitors in hormone-receptor positive BC survivors who are obese may not be as effective as for normal-weight women. In addition, we summarize the findings from studies that show that obesity-induced changes in glucose metabolism, type-2 diabetes, and metabolic syndrome contribute to negative outcomes in BC survivors. Given the evidence, there is a critical need to determine whether weight loss can improve outcomes in BC survivors. © 2013 Springer Science+Business Media New York.