Summary: In 1915 Marie Krogh was the first to describe the diffusing capacity of the lung for carbon monoxide. Today the test is recognized as a valuable screening tool to examine the functional properties of the air-blood barrier in the lungs and has become an essential part of clinical diagnostics in lung diseases. The most frequently used method of assessing the diffusing capacity (lung transfer) for carbon monoxide (DLCO) is the single breath technique. In 2017 a joint task force of the European Respiratory Society (ERS) and the American Thoracic Society (ATS) published an update on DLCO measurement. The assessment of diffusing capacity is most useful in the evaluation of patients with interstitial lung diseases, unexplained dyspnea, obstructive lung diseases and in patients with pulmonary vascular diseases.