Extravehicular Activity (EVA) is an important part of the U.S. space program. The argument that EVA's are unnecessary or too dangerous has been refuted by the successes of contingency and planned EVA's. The recent successes in satellite maintenance and retrieval have demonstrated EVA's to be useful, practical, and safe.During the Space Station Program, crewmembers will be expected to perform more frequent EVA's. As in the past, the physiological factors must be integrated with operational and engineering considerations to achieve a safe, effective system. In past programs, we have been concerned with factors including metabolic work rates and extensive prebreathing methods to rid the body of bends-inducing nitrogen.In the Space Station Program, we are presented with frequent and varied EVA tasks which require that the hardware withstand repeated use, and that physiological limits not be exceeded.