In this multicenter study, we aim to assess the acceptance and effectiveness of a manual based low key group intervention focusing on improving compliance and psychosocial functioning in adolescents with morbid obesity (BMI≥35kg/m2), in comparison to routine care. Youth with more moderate degrees of obesity (BMI 30-34.9kg/m2) will serve as an additional control group. The study is part of the Medical and psychosocial implications of adolescent extreme obesity - acceptance and effects of structured care", short: "Youth with extreme obesity Study (YES), which also comprises the recruitment and characterization of obese (BMI 30-34.9kg/m2) and extremely obese (BMI ≥ 35kg/m2) youth from different healthcare- and non healthcare settings, a structured prospective evaluation of adolescent bariatric surgery, economic assessments of the financial burden of extreme adolescent obesity on the healthcare system, and a long-term prospective observation study.