Increased complexity in organisms narrows the range of body temperature within which effective function is found. The organism responds to this problem by developing both internal and external mechanisms for keeping a steady body temperature independent of the environment. However, there comes a point at which these protective devices break down; the study of the response to arctic conditions involves the study of these three aspects, namely, external protection, internal changes, and the nature of the failure to resist the cold. Bodily functions fail, as does the protective mechanism, from the outside inwards, the vital functions naturally being the last to fail. Recent advances in engineering, particularly developments in aviation, have tended to draw attention to the fact that the limits to arctic activities are usually set by the extent to which man can adapt himself rather than his machines. The adjustments made by people living in very cold regions include a whole number of changes in behaviour which can be grouped under the general heading of the techniques of arctic living. Such changes in behaviour are obviously of the first importance for success and survival, especially when directed to the provision of a satisfactory private climate by the use of special clothing and shelter. Adjustments of a rather different kind from these can also be found in human beings attempting to adapt to arctic surroundings, and it is these other changes in human beings that are being considered here—the physiological and psychological changes that help men to meet the challenge of a cold environment.