In the American Thoracic Society/European Respiratory Society consensus classification, idiopathic interstitial pneumonias are classified into seven clinicopathologic entities. The classification is largely based on histopathology, but depends on the close interaction of clinician, radiologist, and pathologist. An accurate diagnosis can be very difficult, especially when deciding between idiopathic pulmonary fibrosis and fibrotic nonspecific interstitial pneumonia; better diagnostic markers are needed. The prognosis of idiopathic pulmonary fibrosis is very poor, with median survival of 2-4 yr after the diagnosis, yet the course of individual patients is highly variable. Predicting prognosis in the individual patient is challenging but various clinical and radiologic variables have been identified. According to several recent clinical trials, the natural history of this disease may involve periods of relative stability punctuated by acute exacerbations of disease that result in substantial morbidity or death. Nonspecific interstitial pneumonia is characterized by a distinct histopathologic appearance and a better prognosis than idiopathic pulmonary fibrosis. However, there is still confusion and controversy over the relationship between idiopathic pulmonary fibrosis and fibrotic nonspecific interstitial pneumonia.