Obesity affects the risk of breast cancer development and the risk of recurrence and death from breast cancer. Furthermore, weight gain after diagnosis of breast cancer (independent of BMI at diagnosis) is associated with higher all-cause mortality rates compared with maintaining body weight. The American Cancer Society (ACS) recommends cancer survivors achieve and maintain a healthy weight, follow a dietary pattern high in vegetables, fruits, and whole grains, engage in 150 minutes per week of aerobic exercise plus two strength training sessions per week, and avoid physical inactivity. Despite these recommendations, over 65% of breast cancer survivors are overweight or obese, and fewer than 30% engage in recommended levels of physical activity. Recently, the American Society of Clinical Oncology (ASCO) published a position statement on obesity and cancer with a multipronged initiative to reduce the impact of obesity on cancer, with one of the initiatives focused on determining best methods to help cancer survivors make effective changes in lifestyle behaviors. To date, only a few diet- and exercise-induced weight loss trials in breast cancer survivors have been published. We conducted a diet- and exercise-induced weight loss trial in overweight and obese breast cancer survivors, entitled the Lifestyle, Exercise and Nutrition (LEAN) Study, to examine the effect of weight loss on body composition and biological markers related to breast cancer. This study as well as other weight loss trials in women with breast cancer will be discussed, as well as future research and clinical and community-based weight management programs.