 The purpose of this study is to test the effectiveness of a behavioral weight loss and exercise intervention delivered via the internet at different cancer centers in New England. The investigators hypothesize that participants will lose 7 of their body weight. The research is being done because many women who are treated for breast cancer are overweight and treatment for breast cancer is often accompanied by weight gain. Individuals who are overweight may be more likely to have their breast cancer come back. Also women who exercise at least three hours a week seem to have less chance of their cancer coming back. The investigators hope to identify an effective weight loss and exercise intervention for breast cancer survivors that can be easily accessed by many people. 