The importance of maintaining body temperature is well understood by both clinicians and lay persons. The normal, resting body core temperature is about 37 C (98.6 F), and death often occurs when it falls below 27 C (80.6 F) or exceeds 42 C (107.6 F). Thus, for survival the degree of overheating is more critical than that for overcooling. The lower limit of body core temperature for onset of heatstroke is usually only 41 C - 42 C (105.8 F - 107.6 F), but classic heatstroke has occurred with core body temperature as low as 40.6 (105.1 F). The physical effects of weightlessness on heat loss responses have not been well investigated; however, convective heat loss is greatly compromised and evaporative heat loss may also be impaired. These effects on convection and evaporation can be minimized by increasing air flow and reducing ambient humidity. The specific environments that crew members may be expected to encounter are described in this chapter.