Obesity not only is an independent risk factor of postmenopausal breast cancer (BC), and in particular estrogen receptor-positive/progesterone receptor-positive BC, it is also a prognostic factor of the disease. Substantial evidence has shown that obesity, as measured by body mass index (BMI) is linked to BC outcomes. All-cause and BC-specific mortality risk increase for each BMI unit increase in pre- and postmenopausal BC survivors is estimated to range from 8 to 29 %, depending on when BMI is ascertained. The positive associations in pre- and postmenopausal BC and in hormone receptor-positive and hormone receptor-negative BC are not significantly different. Furthermore, the negative impact of abdominal obesity on BC survival highlights the need of using fat distribution (waist circumference, waist-hip-ratio) as well as general obesity (BMI) to evaluate prognosis in the clinical setting. More research is needed to elucidate possible differential associations in pre- and postmenopausal BC that are defined by hormone receptor and/or human epidermal growth factor receptor (HER), and in advanced tumors; for which the data are limited and less clear. Current evidence on treatment toxicity supports the guidelines from the American Society for Clinical Oncology, which recommends the use of full weight-based chemotherapy to treat obese cancer patients. Several studies have shown that lifestyle interventions are feasible and safe; more research is needed on specific diets for health maintenance and weight loss in BC survivors. Being physically active (≥150 min/week of moderate intensity activity) helps manage body weight (normal BMI 18.5-24.9 kg/m(2)), improves survival, and has secondary health benefits. Oncologists should recommend their patients to be physically active and control body weight when the conditions of the patient allow it.