As the global prevalence of childhood overweight and obesity spreads to low and middle income countries, there is an increasing need for researchers to assess overweight and obesity in populations where child undernutrition still prevails. Although BMI (body mass index) cutoffs are widely used in research and project evaluations, they have only recently been included in WHO definitions for overweight and obesity in children. This review describes the history of how and why BMI was introduced as a proxy for adiposity in children, the scientific evidence and examples from epidemiological studies. Overall, BMI continues to be a valuable measure in children if the underlying assumptions of the criteria and cut-off values are considered. However, where BMI is associated with height, in children, we recommend using weight for height z-scores. © 2013 Elsevier B.V.