Despite growing concern about weight-related problems among children, no universally accepted classification system for childhood obesity exists. There is a number of proposed international body mass index (BMI)-based systems in use and national variants also exist in many countries. The absence of a universally accepted standard and confusion concerning which classification system to use on national levels complicate monitoring of the development of the obesity epidemic, stratification for selective interventions in public health, screening in clinical practice and comparisons between studies. Some proposed international classification systems have not only been recommended for global monitoring and comparisons between studies, but also for clinical and national epidemiological use in some countries. Possible discrepancies may thereby lead to inefficiencies in health care delivery and prevention programmes. The problems associated with misclassification of individuals at risk may lead to overconsumption of health care resources by lower-risk individuals and underconsumption by higher-risk individuals, which is costly both in terms of foregone health improvements and in terms of wasteful monetary usage. The aim of this paper was to review the specific problems associated with BMI as a measure of adiposity in childhood, the most commonly used classification systems for childhood obesity based on BMI, and how their performance can be evaluated.