Clinical examples could be multiplied, but they are intended solely to illustrate the application of studies of respiratory function to the management of patients under the care of the anaesthetist. The vast majority of patients can be adequately assessed by the anaesthetist, aided when necessary by the range of simple measuring instruments which are now available. However, there remain a small number of patients whose life is threatened by a disorder of respiratory function which cannot easily be defined without an assessment of ventilation/perfusion relationships based on analysis of arterial blood. The current range of therapeutic procedures is sufficient to enable the anaesthetist to preserve safe levels of arterial\(P_{O_2 } \) and\(P_{CO_2 } \) and so to prevent death from respiratory causes in all but the most extreme forms of disorder. We should therefore be able to assess the specific disorder and so to institute the appropriate form of therapy.