Space Station tours for humans may extend to 180 days. Adaptations of the body's physiological systems occur as a result of prolonged weightlessness, as well as other conditions of space flight. These changes are of various degree and often follow differing time courses depending on the ship environment, crew activity, and individual peculiarities. Because the U.S. space program utilized short duration flights, the opportunity to study these physiological adaptations has been, for the U.S., fairly limited. An understanding of these changes in body function, their mechanisms and their consequences are of obvious impact to assessing crew capabilities and health, both onboard the station, as well as during the rigors of return to Earth, reentry and landing. It will be of considerable importance to consider the human adaptation to space when designing and engineering Space Station.