Water needs, water sources, and means for recycling water are examined in terms appropriate to the water-quality requirements of a small crew and spacecraft intended for Mars exploration. Inorganic, organic, and biological hazards are estimated for wastewater sources. Sensitivities to these hazards for human uses are estimated.The water recycling processes considered are humidity condensation, carbon dioxide reduction, waste oxidation, distillation, reverse osmosis, pervaporation, electrodialysis, ion exchange, carbon sorption, and electrochemical oxidation. Limitations and applications of these processes are evaluated in terms of water-quality objectives. Computer simulation of chemical processes is examined.Recommendations are made for development of new water recycling technology and improvement of existing technology for near-term application to life support systems for humans in space.