Obesity is associated with an increased risk of breast cancer, and increased risk of recurrence in women who develop breast cancer. Evidence suggests that the risk of estrogen-receptor (ER)-positive breast cancer is increased in obese postmenopausal women, whereas in premenopausal women the risk of triple negative breast cancer is increased. Nonetheless, the presence of obesity at diagnosis, and possibly weight gain after diagnosis, may independently contribute to an individual's risk of recurrence of both pre- and postmenopausal breast cancer. Factors associated with adiposity that are likely contributing factors include hyperinsulinemia, inflammation, and relative hyperestrogenemia. Some studies suggest that some aromatase inhibitors may be less effective in obese women than lean women. Clinical trials have evaluated pharmacologic (eg, metformin) and dietary/lifestyle interventions to reduce breast cancer recurrence, although these interventions have not been tested in obese women who may be most likely to benefit from them. Further research is required in order to identify adiposity-associated factors driving recurrence, and design clinical trials to specifically test interventions in obese women at highest risk of recurrence.