Idiopathic interstitial pneumonias (IIP) are a relatively common cause of diffuse lung disease marked by varying degrees of inflammation and fibrosis within the lung, leading to substantial morbidity and mortality. Knowledge and progress in therapeutics have been hindered by inconsistent definitions and challenges in diagnosis. Present classification schemes are based on morphologic definitions that frequently require synthesis of clinical, radiologic and histopathologic information. This review focuses on the present state of diagnosis, and the role that history, radiographic imaging and surgical lung biopsy have on the diagnosis of IIPs. Surgical lung biopsy plays a key role in distinguishing specific IIPs in patients whose imaging studies lack features diagnostic of usual interstitial pneumonia. The final diagnosis is based on the combined clinical, radiographic and histologic findings, and is best accomplished using a dynamic, multidisciplinary approach. More accurate diagnoses should increase the ability to study and understand the biology, natural history and potential therapies for this group of devastating diseases.