The diffusing capacity for carbon monoxide (DL(CO)) is a commonly performed and clinically useful pulmonary function test that provides a quantitative measure of gas transfer in the lungs. It is valuable for evaluating and managing patients with a wide variety of pulmonary disorders, especially patients with interstitial lung disease, pulmonary vascular disease, and obstructive lung disease. Important aspects of the DL(CO) test are discussed including the physiologic principles governing diffusion, testing technique and equipment, technical and physiologic factors influencing DL(CO) variability, DL(CO) test interpretation, and the clinical utility of DL(CO) measurement.