Temperature has pronounced and complex effects on cellular physiology. Rates of enzymatic processes display an exponential change with temperature, as expressed by the Q10 relationship. The basis of these effects may be temperature induced phase transitions in membrane lipids and protein associated water, effects on bulk water and effects on the relationship between water and inorganic solutes. Hypothermia may be lead to a collapse in ionic regulation, leading to an uncontrollable and lethal calcium influx. Subfreezing temperatures may cause injury due to cellular freezing with subsequent excessive osmotic swelling, lyotropic effects or excessive osmotic shrinking due to extracellular freezing. Cells may protect themselves by freeze avoidance accomplished by removal of ice nucleators, production of proteinaceous antifreeze agents and accumulation of polyols. Alternatively they may secure extracellular freezing by production of extracellular ice nucleating agents, and counteract lyotropic effects and osmotic shrinking by accumulation of polyols which reduce ice content in a colligative manner.