Regarding the fact that the ventilation of most underground systems is mainly passive and mostly forced by the movement of incoming and outgoing trains as well as natural exchange processes through relatively small openings of entrances and exits and sometimes vent shafts, the occurrence of situations of thermal discomfort to the subway users seem to be very likely. Temperature measurements from the New York City Subway and the Washington D. C. Metro as well as in German Subway Systems showed amazing results. Besides air temperature, other factors have to be taken into consideration to prove the occurrence of such situations of discomfort or stress to the human organism. Thinking of an underground system like a subway station the influencing factors on the human body are quite different to the situation in the outer atmosphere. For example, solar radiation is reduced to thermal radiation emitted from walls, floors, ceilings and the surplus heat of trains, air condition of work- and equipment rooms as well as from subterranean community heating systems. This field can be of high importance as the underground environment provides only restricted ventilation. To identify the thermal situation temperature measurements were conducted in four different subway systems . Many manual measurements in different subway stations were made to record the spatial pattern of air temperature while installed measurement stations were used to determine the course of air temperature in both tunnels and stations. The measurements show significant differences between outside air temperature and air temperatures in the station (up to 25K). Moreover, air temperature varies to an amazing extent inside the station itself (up to 20K). Within the subway station, maximum air temperatures of up to 40°C were recorded; the maximum of radiation temperature observed at some walls was 50°C.