Quantitative computed tomography is being increasingly used to quantify the features of chronic obstructive pulmonary disease, specifically emphysema, air trapping, and airway abnormality. For quantification of emphysema, the density mask technique is most widely used, with threshold on the order of-950 HU, but percentile cutoff may be less sensitive to volume changes. Sources of variation include depth of inspiration, scanner make and model, technical parameters, and cigarette smoking. On expiratory computed tomography (CT), air trapping may be quantified by evaluating the percentage of lung volume less than a given threshold (eg, −856 HU) by comparing lung volumes and attenuation on expiration and inspiration or, as done more recently, by coregistering inspiratory and expiratory CT scans. All of these indices correlate well with the severity of physiological airway obstruction. By constructing a 3-dimensional model of the airway from volumetric CT, it is possible to measure dimensions (external and internal diameters and airway wall thickness) of segmental and subsegmental airways orthogonal to their long axes. Measurement of airway parameters correlates with the severity of airflow obstruction and with the history of chronic obstructive pulmonary disease exacerbation.