It has long been supposed that repeated exposure of a man to cold conditions might be expected to produce changes within him which would better enable him to stand up to those cold conditionsto acclimatize to cold. Until recently, however, there has been little and conflicting evidence of the mechanism of such changes. Accounts of the polar explorers of the Scott-Amundsen era reveal that these men were exposed to severe cold stresses, but unfortunately the sophisticated research facilities necessary to measure the various factors involved were not then available. Now that these facilities are available, man conducts his polar activities under far more comfortable conditions, and the degree of cold stress to which he is exposed is probably of a much smaller order. Small wonder, then, that research into the elusive phenomenon of acclimatization to cold has yielded little profit. In fact, the available evidence suggests that the polar traveller may well undergo a marked heat stress as a result of strenuous exercise under conditions in which he is unable to get rid of surplus heat easily. This article looks at the basic physiological problems of heat loss and heat conservation with which the modern polar traveller has to contend, and points out the results of some recent investigations.