Idiopathic interstitial pneumonias (IIP) comprise seven entities with distinct histologic patterns. In their idiopathic form IIP are rare diseases. They are, nevertheless, considered prototypes of the much more common secondary interstitial pneumonias. The advent of high-resolution computed tomography (HRCT) has had a profound impact on the imaging of IIP, because the detailed delineation of the lung anatomy allows a close correlation between the histologic patterns of IIP and the CT features. On the basis of CT morphology and in the correct clinical context, the radiologist can achieve an accurate diagnosis in many cases. However, due to overlap between the various entities, complementary lung biopsy is recommended in virtually all cases. This article reviews the CT pattern of IIP and offers relevant clinical and histological information for the purpose of enabling the radiologist to understand and participate in the multidisciplinary concept of IIP.