In June 2001, using an "international multidisciplinary consensus," the American Thoracic Society and European Respiratory Society classified the idiopathic interstitial pneumonias (Am J Respir Crit Care Med 2002;165:277-304). The American Thoracic Society and European Respiratory Society coined the term idiopathic interstitial pneumonias because the diseases in question have both unknown etiologies and various combinations of inflammation and fibrosis within the lung parenchyma. The consensus included the development of a classification standard of the interstitial pneumonias and discussed the role of surgical lung biopsy and bronchoalveolar lavage evaluation. The classification of idiopathic interstitial pneumonias is based on histologic criteria, which correlate to imaging patterns on high-resolution computed tomography. The group of lung diseases under the umbrella of idiopathic interstitial pneumonias includes the following: usual interstitial pneumonia, nonspecific interstitial pneumonia, desquamative interstitial pneumonia, respiratory bronchiolitis interstitial lung disease, acute interstitial pneumonia, cryptogenic organizing pneumonia, and lymphoid interstitial pneumonia.