Human locomotion is an essential capability for normal activity on Earth. It requires the largest muscle and bone mass in the body and this muscle activity normally determines cardio-respiratory capacity. Active individuals, including the astronaut population, usually take > 104 steps day - 1 at rates of 1 - 3 steps sec - 1 with foot ground forces of 1 - 1.4 body weight (B.W.) walking and 2-3 + B W. running, producing up to 300W equivalent work. Forces and energy levels are a function of mode (walk or run), B.W, velocity, and surface grade and consistency. Overground and treadmill (T.M.) locomotion are equivalent on Earth.In weightlessness, locomotion is impossible and, without exercise, there will be a rapid loss of muscle, bone, and cardio-respiratory capacity such that, after several months' spaceflight without exercise, most individuals will be unable to walk from strength loss. Currently, the only exerciser capable of providing locomotor activity in flight is a treadmill which must also provide a constant axial truncal force equivalent to B.W. Subject-driven treadmills can operate at a variety of controlled speeds but only at some minimum equivalent grade, and a motor-driven unit can be designed to operate over all practical speeds and equivalent grades. Both US and USSR programs have simple flight T.M.s. US T.M.s and instrumentation have been designed and prototypes demonstrated that provide performance equivalent to Earth units. Such designs, if properly utilized, should maintain locomotor and cardio-respiratory capacity during long flight at a level consistent with required postflight activities.