Idiopathic pulmonary fibrosis (IPF; also known as cryptogenic fibrosing alveolitis) is a distinctive type of chronic fibrosing interstitial pneumonia of unknown cause associated with the histological pattern usual interstitial pneumonia (UIP). UIP is a distinct histological pattern observed in IPF but may also be found in other etiologies. The diagnosis of UIP can be established by surgical lung biopsy or by high-resolution thin-section CT scans (provided the radiographic features are classical). Historically, patients labeled as 'IPF' encompassed a group of disorders, including UIP, as well as other idiopathic interstitial pneumonias, which differ from UIP in prognosis and responsiveness to therapy. The term IPF should be restricted to patients with idiopathic UIP. The inciting cause(s) and pathogenesis of IPF have not been elucidated but alveolar epithelial cell injury and dysregulation or altered phenotypic expression of fibroblasts are key elements. Inflammatory cells may play minor roles in initiating or propagating the fibrotic process. The prognosis of idiopathic UIP is poor. Mean survival following diagnosis approximates at 3 years. Current medical therapies are of unproven value. Lung transplantation is a viable option for patients failing medical therapy.