Among the computed tomography (CT) signs of parenchymal lung disease, the ground-glass pattern is the one most difficult to diagnose and most influenced by CT technique. Ground-glass opacity may result from changes in the airspaces or interstitial tissues in acute or chronic infiltrative lung disease. It may also be seen as a consequence of increased capillary blood volume in redistribution of blood flow due to airway disease, emphysema, or pulmonary thromboembolism. Definition of this sign on high-resolution CT (HRCT) images, its various HRCT patterns, and potential pitfalls in its recognition are described with special attention to optimal HRCT technique.