Due to the need for human beings to adapt their heat budget to the thermal environment in order to optimise comfort, performance and health the adaptation issue is a question of vital importance. Balancing the human heat budget, i.e. equilibration of the organism to variable environmental (atmospheric) and metabolic heat loads is controlled by a very efficient (for healthy people) autonomous thermoregulatory system that is additionally supported by behavioural adaptation which are driven by conscious sensations of thermal discomfort. These capabilities enable the (healthy) human being to live and to work in virtually any climate zone on earth, albeit with varying degrees of discomfort. Based on mortality studies a large number of publications show the evidence of adverse health impacts by thermal stresses, in particular during heat waves. Based on thermo physiology and heat exchange theory an overview is given on different assessment approaches up to the development of the Universal Thermal Climate Index within ISB Commission 6 and the European COST Action 730. Selected applications from the weather/climate and human health field such as Heat Health Warning Systems HHWS and precautionary planning in urban areas illustrate the significance of thermal assessments with respect to short-term and long-term adaptation. A huge potential to save energy-and by this to avoid CO2 emissions-without loosing acceptable thermal conditions indoors, also in a future warmer climate, results from a adaptive model which has been derived from thermal comfort investigations across the world.