The multidisciplinary guideline 'Diagnosis and treatment of obesity in adults and children' developed by the Dutch Institute for Healthcare Improvement (CBO) is based on published scientific evidence whenever possible. Diagnosis ofobesity requires a body-mass index (BMI) of 30 kg/m2 or more with additional assessment of waist circumference and comorbidity. For children and adolescents, use ofage-specific BMI thresholds is recommended. Treatment of obesity consists of multiple lifestyle interventions for at least 1 year, followed by long-term management aimed at weight maintenance or any further weight loss. In adults, the goal is to achieve weight loss of at least 5% and a reduction in waist circumference of at least 10% after 1 year. If weight loss after 1 year is less than 5%, the addition of pharmacological interventions to lifestyle interventions can be considered. Bariatric surgery can be considered for patients with a BMI of 40 kg/m2 or more and for those with BMI 35-40 kg/m2 with one or more comorbidities. Pharmacological and surgical interventions are not recommended for children and adolescents.