Pulmonary emphysema is a pathological diagnosis. The clinical diagnosis of emphysema can be difficult because correlations between results of lung function tests and the extent of emphysema are poor. Features of chronic bronchitis or asthma may overlap with emphysema, making the clinical diagnosis more challenging. Nonetheless, the diagnosis of pulmonary emphysema can be made with relative confidence on the basis of clinical and radiological criteria. Despite not detecting mild emphysema and underestimating the severity of disease, CT--and high-resolution CT in particular--is the best noninvasive modality for detecting or corroborating pulmonary emphysema. This review focuses on several important aspects of pulmonary emphysema: (1) the definition and pathological characterization, (2) techniques of CT imaging, (3) CT findings and their correlation with pathophysiological data, and (4) quantification with CT.