The purpose of this randomized controlled trial is twofold. First, to examine the efficacy of a 24-session, 6-month family-based behavioral intervention (FBBI) - as compared to a waitlist FBBI group, which later receives the same FBBI - that targets weight loss in adolescents/young adults with intellectual disability aged 14-22 years. Second, to examine the efficacy of a 12-session, 6-month Maintenance intervention that targets maintenance of weight loss in the same population of adolescents/young adults with intellectual disability. The Maintenance condition follows the completion of each FBBI group and involves a re-randomization to either the Maintenance intervention or no further intervention. Primary outcome measures include body weight and Body Mass Index (BMI). Secondary outcome measures include physical activity/sedentary behavior (measured via accelerometry), dietary patterns (3-Day Food Records), and self-efficacy (brief questionnaire). Hypotheses are that: (1) participants in the FBBI condition will lose more weight (and reduced BMI) than participants in the waitlist treatment condition, and that (2) participants in the Maintenance condition will maintain weight lost (and reductions in BMI) or experience less weight regain, as compared with participants who receive no further intervention following FBBI.

Inclusion Criteria: Inclusion criteria for the adolescents with intellectual disability (ID) will be:

1. Age 14-22 years
2. Clinically overweight or obese, as indicated by BMI guidelines set by the Centers for Disease Control and Prevention
3. Scores of ≤ 75 on the Kaufman Brief Intelligence Test (KBIT) and Vineland Adaptive Behavior Scales, 2nd Edition to establish criteria for the presence of an intellectual disability
4. Living at home in a single- or two-parent family with no plans to leave home in the next year
5. Verbal ability and necessary behavioral/social skills to participate in a group classroom-based educational program (determined by clinical observation and judgment accomplished through a structured interview during enrollment)
6. Signed approval to participate by the adolescent's primary care provider (and by other specialists such as cardiologists or neurologists if the participant has a heart condition or neurological disorder).

Exclusion Criteria: Exclusion criteria will be:

1. Cardiac problems that preclude participation in moderate-to-vigorous-physical-activity (MVPA)
2. Insulin-dependent diabetes
3. An active seizure disorder not stabilized on medication
4. Non-ambulatory, i.e. uses a wheelchair, or orthopedic injuries/deformities that prohibit moderate walking and other exercise
5. Colitis, Crohn's Disease, and Inflammatory Bowel Disease
6. Prader Willi syndrome
7. Unwillingness to wear the accelerometer at screening or enrollment
8. Recent history of disruptive, inappropriate or dangerous behaviors (e.g., self-injury, aggression/injury to others, property destruction, and extreme and ongoing use of inappropriate language).