Extravehicular activity (EVA) has become increasingly complex since the days of Gemini. Crewmembers may accumulate as many as 250 hours EVA during a 90 day mission. Physiological parameters and operational variables which were of little or no concern on Shuttle EVAs may become major factors for Space Station EVAs in terms of limiting man's productivity and thus impact EVA scheduling, tasks, and safety. Repeated decompressions, suit oxygen and carbon dioxide levels, metabolic requirements for optimization of work, thermal balance and comfort, and waste collection and management are discussed in this paper. The physiologist must determine the limits of man's adaptation to the space environment within the context of defined, measurable parameters of work performance, or define the change in performance when given an altered enviornment as the independent variable.