The idiopathic interstitial pneumonias (IIPs) are a group of diffuse parenchymal lung diseases of unknown etiology characterized by the presence of various degrees of inflammation and fibrosis. Confident definitive diagnosis of the various IIPs requires dynamic interaction among clinicians, radiologists, and pathologists to arrive at a clinico-radiologic-pathologic diagnosis. The aims of this manuscript are to summarize the characteristic clinical and histologic manifestations, and to describe and illustrate the high-resolution computed tomography manifestations of the IIPs. The focus will be on idiopathic pulmonary fibrosis (idiopathic usual interstitial pneumonia), nonspecific interstitial pneumonia, cryptogenic organizing pneumonia, and acute interstitial pneumonia. High-resolution computed tomography plays an important role in the initial diagnosis, the assessment of disease extent, the likelihood of response to treatment and prognosis, and the assessment of complications.