One of the major concerns for both short- and long-duration spaceflight is the phenomenon of cardiovascular deconditioning. Exercise deconditioning during spaceflight may significantly affect a crewmember’s ability to perform strenuous or prolonged tasks during and after a spaceflight mission, respond to an emergency situation, or assist a crewmate who might be incapacitated. This chapter introduces the principles of cardiovascular fluid and electrolyte control to shed light on the symptoms typically reported by astronauts during and after spaceflight. Data from flight experiments are discussed, as well as the value of ground-based models such as bed rest studies. The value of exercise, inflatable suits, saline loading, and artificial gravity is also discussed.