Summary The demand for oxygen for industrial processes which has been steadily growing since the liquefaction method of air separation was introduced fifty years ago has, until recently, been met by extension of the “classical process” and especially by the production of liquid oxygen on a large scale. The use of liquid oxygen is particularly convenient when the demand is highly irregular and intermittent, as, for instance, in steel-works practice, but there is an increasing demand for oxygen in chemical and metallurgical processes which operate continuously and steadily, and which require oxygen of only moderate purity. When the demand for such purposes is large oxygen is best produced in gaseous form in plants operating at low pressure, and the term “Tonnage Oxygen” has been applied to plants of this type, which supply oxygen in quantities sometimes of several hundred tons a day. The principles of low temperature air separation are discussed from the point of view of thermodynamic efficiency and the developments which have made possible the low-pressure processes of “Tonnage Oxygen” production are outlined. A brief survey is made of the uses of oxygen in metallurgical, gasification, and chemical processes. Oxygen is used in many different ways in the conversion of raw iron to finished steel, and a number of new steel-making processes have been developed which depend on a “Tonnage Oxygen” supply. Many processes based on solid liquid or gaseous fuels make use of oxygen in the preparation of synthesis gas for the manufacture of ammonia, petrol, and organic chemical products. Oxygen is also used in the complete gasification of coal to produce a fuel gas of high calorific value. In a number of applications the use of oxygen confers the advantage that it enables low-grade raw materials to be processed economically, and an increasing demand for these purposes is predicted.