Being small, shapeless and inert a gas molecule does not seem to be an enzyme's dream of a substrate. Nevertheless evolution has provided a host of enzymes which can interact specifically with gas molecules such as oxygen, carbon dioxide, nitrogen, hydrogen etc. Many of these enzymes play dominant roles on the world scene in biogeochemical cycles. On the cellular level they tend to be closely connected to the energy conserving apparatus. We define Gas Enzymology as the study of these enzymes. Historically, Gas Enzymology is a subspecialty of bioenergetics. Its foundations, technical as well as conceptual were laid by Warburg in his studies of the cellular combustion of nutrients. The Warburg apparatus supported the first thirty years of research in the field. It was succeeded by the Clark electrode which had its heyday during the period when the modern concepts of bioenergetics took shape. The Clark electrode, itself approaching thirty years of age, is now being sup­ plemented and in some cases replaced by the vastly more powerful membrane inlet mass spectrometer which measures with equal ease all dis­ solved gases of interest in biochemistry. It is our belief that future development of Gas Enzymology will be linked to the widespread exploit­ ation of this technique.