Careful histopathological evaluation has shown the traditionally clinical diagnosis of idiopathic interstitial pneumonia to be more heterogeneous than once thought. Its subclassification, based on clinicopathological criteria, has important therapeutic and prognostic implications. The most important distinction is the presence of usual interstitial pneumonia, the histopathological pattern seen in idiopathic pulmonary fibrosis. Idiopathic pulmonary fibrosis has a worse response to therapy and prognosis. New insight into the pathophysiology of idiopathic pulmonary fibrosis suggests a distinctly fibroproliferative process, and antifibrotic therapies show promise. Although the clinical and radiographic diagnosis of idiopathic pulmonary fibrosis can be made confidently in some cases, many patients require surgical lung biopsy to determine their underlying histopathological pattern. A structured, clinicopathological approach to the diagnosis of idiopathic interstitial pneumonia, with particular attention to the identification of idiopathic pulmonary fibrosis, ensures proper therapy, enhances prognosis, and allows for further investigation of therapies aimed at the distinct pathophysiology.