Inspired and expired lung volumes measured by spiro- metry have been used clinically for over one hundred years and have proved useful in the detection, characteriza- tion, and quantification of the severity of lung disease. Measurements of absolute lung volumes, residual vol- ume (RV), functional residual capacity (FRC), and total lung capacity (TLC) have been used clinically for fewer years, are technically more challenging, and the results are more controversial regarding their clinical useful- ness. In contrast with the relative simplicity of spirometric volumes, because the measurements of absolute lung volumes include the "inaccessible" space of residual vol- ume, a variety of techniques have been developed to make these measurements. These include gas dilution (as used in the helium or argon dilution technique), gas washout (as used in the nitrogen washout measure- ments), body plethysmography using a variety of spe- cific methodologies, and radiographic imaging using standard posterior-anterior and lateral chest radiographs and computerized tomography. As part of the ongoing efforts of the American Thoracic Society (ATS) to develop recommendations and stan- dards regarding clinical testing, a workshop was con- vened and charged with developing recommendations for measuring absolute lung volumes in humans. In order to provide the diversity of expertise necessary to meet these challenges, and with the goal of developing rec- ommendations that could be endorsed by the European Respiratory Society (ERS) as well as the ATS, many of those invited to participate in the workshop were from Europe. It was recognized that these measurements often pre- sented different challenges in infants and children; how- ever, we also concluded that, in the workshop, it would be useful to integrate those with experience in making these measurements in both adult and pediatric envi- ronments, not only with the goal of developing more universally applicable recommendations and "standards", but also because we believed that exchanges of infor- mation about the measurements unique to either adult or paediatric population might be helpful for better defin- ing optimal techniques and clinical limitations for both groups of subjects. Optimal predictive normal values for lung volumes may be as important as optimal measurement techniques. For this reason we also included a review of normal