Space flight is a new experience for man. Tension on the weight-bearing components of the musculo–skeletal system is greatly reduced, as is the work required for movement. The body responds by a reductive remodelling of the musculo–skeletal system. Protein is lost from muscles with anti-gravity functions. The rate of Ca loss from the weight-bearing bones is about 1 % per month. Voluntary dietary intake is reduced during space flight by about 20 %. These adaptations to weightlessness leave astronauts ill-equipped for life with gravity when they return to earth. Rates of energy expenditure are similar to that expected on the ground for comparable activities. Protein intake is adequate in flight but may be limiting after space flight due to substrate competition between repleting muscle and other anabolic processes. The most serious nutritional problem is the inability to maintain energy balance on missions with high exercise requirements. The poor dietary intake is probably a consequence of engineering-imposed environmental constraints. The low levels of lighting in the space vehicle may not be enough to promote vitamin D synthesis. Nevertheless, the evidence suggests that a normal well-balanced diet with plenty of fluids will be as healthy in space as on earth. The long-term goal of the manned space programme is to develop the means of sustaining human life beyond earth. This will involve the development of technologies to grow food, maintain a breathable atmosphere and recycle waste products with the only external input being energy.