Body mass index (BMI), calculated as weight in kilograms divided by height in meters squared, is used extensively to characterize excessive body weight in adults and children. The Centers for Disease Control and Prevention (CDC) tracks changes in the national prevalence of obesity with BMI, and a report by Ogden and colleagues1 in this issue of JAMA presents the latest data involving children. Recently, a consensus statement prepared by an expert committee, comprising professionals from 15 health care organizations, described BMI as the best available clinical tool to screen for childhood obesity and monitor progress with treatment.2 The statement encourages primary care clinicians to assess obesity risk at all well-child visits using BMI-for-age percentiles. This assessment paradigm has extended into schools, and several states now mandate use of BMI to identify overweight or obese children and evaluate the effectiveness of healthful lifestyle initiatives. However, despite its widespread use, BMI continues to cause confusion among patients and practitioners, raising several fundamental questions.