In brief: Athletes at many levels undergo body composition assessments to learn how much fat they have. Yet the subject of body composition remains fraught with confusion and unanswered questions. How accurate are the present methods for assessing body composition? Of what use is body composition assessment in preventive health care? How much do primary care physicians need to know about body composition? What are the optimal proportions of body fat for athletic performance and for health? How much is known about the measurement of muscle and bone as body components? Four panelists with long experience in body composition research address these and other questions.