Childhood obesity is defined as excessive fat accumulation which presents a risk to health in a person under 18 years old. Measuring fatness is not easy and hence for practical reasons, indirect ways of measuring this using weight-and-height-based anthropometric indices are used e.g. percentage of ideal weight for height (PWH) or body mass index (BMI) as surrogate measures for adiposity. For screening, diagnosis, and monitoring childhood obesity, several countries have already developed country-specific age-and-sex-specific BMI reference charts for their own paediatric populations; these reference values are more sensitive (missing fewer true overweight or obese individuals) than those based on an “international” dataset such as the IOTF-BMI reference values. Diagnosis of obesity should use the BMI percentile with national reference data (e.g., BMI 95th percentile or greater). Epidemiologically, paediatric obesity should be considered a chronic disease with numerous possible co-morbidities. An optimal BMI needs to be aimed for control and prevention. Concerted social policies to promote physical activity and decrease excessive food intake are also needed.