Measurements have been made of cambium temperatures in living trees subjected to mild and fierce fires. Fire resistance depends upon bark thickness: thus only big trees with thick bark can survive unharmed in really severe fires. The rate of change in cambium temperature is related to the thermal diffusivity of the bark, and is largely independent of bark structure or moisture content. A simple mathematical model is used to calculate heat transfer in bark, and the results obtained agree well with experimental measurements made on a variety of trees. Studies of soil temperature and results from experiments on pines are referred to briefly in appendices.