The landscape of childhood health and disease has changed over the past century, and school nurses are now in a unique position to address the conditions that lead to chronic disease, such as obesity. Measuring body mass index (BMI) during childhood and adolescence is the recommended method for screening and/or monitoring obesity in school communities. Yet changes in the size, proportion, and distribution of fat mass and fat-free mass during growth and development introduce challenges to interpreting BMI measurements. Understanding these challenges and ensuring accurate measurement techniques are the foundation for implementing school-based BMI measurement programs. This article will provide an overview of body composition during childhood and adolescence, introduce strategies to improve the accuracy of BMI measurements, and explore the school nurse's role in BMI surveillance and/or screening activities.