Technical Abstract: Hydrodensitometry (underwater weighing) is considered a reference assay for body composition. Our aim was to assess air-displacement plethysmography (ADP) as a replacement for hydrodensitometry (UWW) in children and adults. ADP and UWW measurements were obtained for 51 adults (37 F, 14 M) and 26 children (14 F, 12 M). Both techniques provide an estimate of the body's volume (Vol), which is converted to whole-body density (Den) in order to calculate body fatness (%Fat) expressed as a percentage of body weight. The UWW and ADP results were highly correlated for Vol (r2 = 0.999, SEE = 0.59 L), Den (r2 = 0.864, SEE = 0.0073 g/cc), and %Fat (r2 = 0.870 , SEE = 3.2%). Results of Bland-Altman analyses showed that the differences (bias) between the two methods were not significant for Vol (-0.06 ± 0.59 L ), Den (-0.0006 ± 0.0073 g/cc ), or %Fat (0.3% ± 3.2% ) for the total population. No significant differences among the three parameters were detected for gender or ethnicity, however, a trend for a small bias (p < 0.06) was observed between adults and children, with a poorer agreement for children. We conclude that ADP is an adequate replacement for UWW for healthy adults, but additional refinements may be needed to achieve interchangeability between ADP and UWW for children.