The amounts of fat and fat-free mass (FFM) are functions of lifestyle, diseases, increasing age, and genetics. The levels of these body compartments are established risk factors for cardiovascular and related chronic diseases. Body composition can be assessed by several methods. The measurements for each method have inherent variations, which can be due to error in the measurement or biological variation. This presentation focuses on the reliability and precision of body composition measurements and the impact of these errors on epidemiological and population-based studies. The effects of these errors in applying body composition to epidemiological studies include the following: (1) the association of cardiovascular risk factors with body composition in cross-sectional studies; (2) changes in body composition in longitudinal or intervention studies; (3) association of changes in body composition with cardiovascular risk factors; and (4) comparison of levels of body composition among subgroups in cross-sectional studies and in intervention studies.