 The purpose of this study is to compare three methods of delivering behavioral weight loss treatment.Participants are randomly assigned to one of three 18-month weight loss treatments group, individual, or smartphone treatment. Participants who are assigned to group treatment will attend group meetings weekly for the first 6 months, then every 2 weeks for 6 months, and then monthly for the final 6 months. Participants who are assigned to individual treatment will attend one-on-one sessions with a weight loss counselor every month for 18 months. Participants who are assigned to smartphone treatment will use a smartphone to learn and practice weight loss skills. They will also attend one-on-one sessions with a weight loss counselor every month for 18 months. Assessment visits are conducted at baseline, 6, 12, and 18 months. 