Body composition is labile and changes as a result of chronic disease, such as metabolic or wasting disorders, and disability. In addition to direct anatomical and physiological effects, the changes in lifestyle associated with such conditions also impact upon body composition. There is a growing interest to understand and evaluate the body composition of individuals with chronic diseases and disabilities. However, for these population groups special attention is warranted since the disease pathology, disability or treatment may alter the basic underlying body composition assumptions and measurement techniques that have been described previously.