Body composition assessment is an important aspect in evaluating health, nutritional status and physical fitness. The proposed research is designed to investigate methodological approaches to assess body composition throughout the life-cycle and to provide normative data on children and older adults which presently is unavailable. In the first aspect of the program, both cross-sectional and longitudinal research designs will be used to extend previous work on children and to initiate new, but related, research focusing on body composition and the adult aging process. Previous research has demonstrated that the fat-free body (FFB) composition varies substantially due to changes in water and bone mineral content during pediatric growth and development. Moreover, there is evidence, although sparse, that the FFB composition changes during he adult aging proces. Two studies are proposed to investigate the integrity of assumptions associated with FFB constancy. The first will follow high and low physically active children (age 10-13 years) longitudinally over four years to validate and extend our past findings based on normal children with respect to growth changes in FFB composition. The second study will investigate change in FFB composition during the adult aging process utilizing a cross-sectional design in a large sample of males and females, 20-70 years of age. To study the constancy of the FFB, a previously developed methodological approach will be utilized involving measures of density (hydrostatic weighing), total body water (deuterium oxide dilution, bone mineral content (photon absorptiometry), and body potassium (40K count). A second aspect of the proposed research program on body composition methodology is the development of measurement techniques which can be applied in clinical examinations and large field studies. Several studies are proposed to investigate the precision with which anthropometric and bioelectric techniques (total body electromagnetic conductivity and whole body electrical resistance) can estimate fat and FFB: 1) a large cross- sectional sample of males and females ages 10-70 years; 2) longitudinal studies to determine the precision of estimating body composition changes during growth and aging; and 3) a study to assess the usefulness of these techniques to measure changes during weight gain and loss. The research plan has been revised to longitudinally study the precision with which anthropometric and bioelectric techniques can track change in body composition during growth and aging. Dual photon absorptiometry has been added to measure total body bone mineral.