Obesity is now the most common disorder of childhood in the developed world, and its prevalence is still increasing. A large body of high-quality and consistent evidence shows that it is best defined using the body mass index (BMI) percentile relative to national BMI reference data. This definition diagnoses excessive fatness adequately, and denotes increased risk of adverse health outcomes. Future research may provide improved obesity definitions for epidemiological use, so that the obesity epidemic can be monitored more effectively. Paediatric obesity causes ill health in both childhood and adulthood, though further research is required on the economic consequences, on some of the co-morbidities in childhood (notably psychological morbidity), and in adulthood where the amount of empirical evidence on long-term effects is limited. The combination of high prevalence with adverse consequences has created a public health crisis.