Non-alcoholic fatty liver disease (NAFLD) has become the most prevalent chronic liver disease worldwide, paralleling the obesity pandemic. Secondary to increasing rates of obesity in children and adolescents, the prevalence of NAFLD has more than doubled in the last decades and is now the most common pediatric liver disease. At present, lifestyle modification by dietary intervention and increasing physical activity is the mainstay of treatment for pediatric NAFLD. Several studies have shown that lifestyle intervention and weight loss improve non-invasive markers of NAFLD. To the investigator's knowledge, data on fibrosis regression following lifestyle treatment in children and adolescents were lacking. The investigators therefore performed a prospective cohort study to investigate the impact of residential lifestyle treatment on liver steatosis and fibrosis in obese children and adolescents. As a follow-up, the investigators now aim to compare these findings with a cohort of well-characterized patients undergoing multidisciplinary, yet ambulatory, weight loss treatment. As such, the investigators will compare the outcomes in two prospective patient cohorts in this non-randomized observational study.