Methods of measuring lean body mass have evolved greatly in the past decade. Because all practical methods of measurement are indirect (i.e., not involving carcass analysis), appropriate models of body composition must be applied. Today's improved precision is due both to better instrumentation and to improved models based on partition of the body into its constituent compartments. With this improved methodology has come the recognition that loss of lean body mass is associated with survival in starvation, acute illness, and aging. In addition, correlations of lean body mass with immune competence and functional status are beginning to emerge. It remains to be proven that changes in lean body mass in aging and disease independently determine outcome. Interventions that preserve or normalize lean body mass could be important for preservation of vigorous functional status.