For many years, the clinical entity of idiopathic interstitial pneumonia (IIP) has been a source of confusion for physicians. There has been much debate over the utility of subclassifying this condition histopathologically. It now appears that such classification is useful, and the most important distinction is the presence or absence of usual interstitial pneumonia (UIP). Unlike the other histopathologic subgroups, UIP has a grave prognosis and responds poorly to traditional therapies. To emphasize this clinical difference, the diagnosis of idiopathic pulmonary fibrosis (IPF), once used synonymously with IIP, is now reserved for only those patients with the histopathologic pattern of UIP. Although the gold standard for the diagnosis of IPF/UIP remains surgical lung biopsy, recent studies suggest that careful clinical and radiographic evaluation can identify IPF/UIP with a specificity of 90% or more. In the absence of a clear clinical diagnosis, we recommend pursuing surgical lung biopsy. Knowledge of the underlying histopathology will allow for more accurate prognosis, help guide therapy, and make possible the clinical investigation of novel therapeutic agents for patients with IIP.