A planning committee of the National Academies of Sciences, Engineering, and Medicine (the National Academies) convened a workshop series to explore the current science on measures of body composition and body fat distribution, with a focus on the strengths and limitations of body mass index (BMI) as a measure of adiposity and an indicator of health. Other topics examined were the anthropological and clinical perspectives of BMI, communicating to different audiences about BMI, and the policy implications of BMI on health care. The series consisted of two workshops held in 2023 (April 4 and June 26). The April workshop explored the science of body composition and body fat distribution measures, with a focus on the strengths and limitations of BMI as a measure of adiposity and health. The presentations explored how different sectors and people from different ethnic groups, cultures, and life stages perceive and use BMI. Workshop presentations also examined BMI and alternative measures to assess obesity morbidity and mortality and how their accuracy affects obesity prevention, treatment, and policy.