Ground-glass opacity of the lung parenchyma is a frequent but nonspecific finding on high-resolution computed tomography (CT) scans. When there is a question of active alveolitis, the radiologist can, however, provide useful clinical information in many cases. Ground-glass opacity reflects morphologic changes beyond the resolution of modem CT scanners which makes a basic understanding of lung anatomy and pathophysiology essential. This review discusses the clinical significance of ground-glass opacity in certain diseases and provides a practical approach to radiologic interpretation based on disease patterns.