NASA: The authors present a physiological basis for the use of exercise as a weightlessness countermeasure, outline special considerations for the development of exercise countermeasures, review and evaluate exercise used during space flight, and provide new approaches and concepts for the implementation of novel exercise countermeasures for future space flight. The discussion of the physiological basis for countermeasures examines maximal oxygen uptake, blood volume, metabolic responses to work, muscle function, bone loss, and orthostatic instability. The discussion of considerations for exercise prescriptions during space flight includes operational considerations, type of exercise, fitness considerations, age and gender, and psychological considerations. The discussion of exercise currently used in space flight examines cycle ergometry, the treadmill, strength training devices, electrical stimulation, and the Penguin suit worn by Russian crews. New approaches to exercise countermeasures include twin bicycles, dynamic resistance exercisers, maximal exercise effects, grasim (gravity simulators), and the relationship between exercise and LBNP.