Bariatric surgery is now tried out as a treatment option for adolescents with severe obesity where other treatment options have not been successful enough. The teenage years are characterized by intense psychosocial development and it is important to know how mental health is affected by undergoing weight loss surgery while an adolescent. In this thesis, mental health is studied in 88 adolescents aged 13–18, before and four months, one year, and two years after undergoing gastric bypass. For a majority of adolescents, mental health is improved both four months and two years after undergoing gastric bypass. Improvements in mental health take place during the first year after surgery, and the second year is characterized by stabilization. However, not all adolescents have a positive mental health outcome. Four months after surgery, 16% of the adolescents report impaired mental health compared to baseline, and two years after surgery one out of five adolescents reports depressive symptoms in the clinical range. The studies indicate that, from a psychological perspective, adolescents with severe obesity undergoing bariatric surgery are a vulnerable group, also in comparison to adults undergoing bariatric surgery. This thesis shows that it is necessary to offer psychological monitoring and interventions to adolescents undergoing bariatric surgery, and that follow-up after the first postoperative year is important. The studies are part of the Swedish national Adolescents Morbid Obesity Surgery (AMOS) study.