Idiopathic interstitial pneumonias are currently classified into four categories of disease: usual, desquamative, and acute interstitial pneumonia, and nonspecific interstitial pneumonia and fibrosis. Usual interstitial pneumonia appears on high-resolution CT (HRCT) as patchy subpleural areas of ground-glass opacity, irregular lines, and honeycombing. Desquamative interstitial pneumonia presents as patchy subpleural areas of ground-glass opacity in middle and lower lung zones. Acute interstitial pneumonia presents as extensive bilateral airspace consolidation and patchy or diffuse bilateral areas of ground-glass opacity. Nonspecific interstitial pneumonia and fibrosis appears as patchy or diffuse areas of ground-glass opacity with associated areas of consolidation and irregular lines. In a subset of patients with diffuse lung disease (especially in those with chronic interstitial lung disease), accurate diagnosis can be made with HRCT findings only, without surgical biopsy. However, HRCT provides a lower level of confidence in the diagnosis of acute or subacute interstitial lung disease such as infection, diffuse alveolar damage, drug reaction, or hemorrhage. Additional expiratory HRCT scans and scans with patients prone help to narrow the differential diagnosis among various diseases and help diagnose or exclude subtle disease in the posterior part of the lung, respectively. HRCT provides a reproducible method for evaluating the global extent of disease. It also discriminates between fibrotic and reversible inflammatory diseases.