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.

Inclusion Criteria:

• Body Mass Index (kg/m2) of 25-45.
• English language fluency
• Ability to read at a 6th grade level
• Availability to attend weekly treatment sessions in Providence, RI

Exclusion Criteria:

• Any heart condition that limits ability to participate in physical activity
• Chest Pain
• Any cognitive of physical limitation that would preclude use of a smartphone
• Serious mental illness
• Historical or current eating disorder
• Previous or planned bariatric surgery
• Use of weight loss medication
• Recent pregnancy or a plan to become pregnant in the next 6 months
• Participation in a study at the Weight Control and Diabetes Research Center within the last two years
• A weight loss of greater than 5% body weight in the last 6 months