Diffusing capacity of the lung for carbon monoxide (DLCO) is a valuable pulmonary function test that can guide diagnostic evaluation, treatment strategies, and prognostic assessment in clinical practice. The test examines the underlying pathophysiology guiding transfer of oxygen from alveoli into the blood. Measurement of DLCO requires patients to perform a simple maneuver of inhalation and breath holding, and results can be influenced by both testing and patient factors of which clinicians should be aware. Gas transfer measurement captures 2 components, alveolar volume and the transfer coefficient, that can further distinguish the many underlying processes that result in DLCO impairment.