Several quite different waste-water processing subsystems have been tested for space mission applications. Mission choice, and choice of priority for further development, involves processor requirements for (electrical) power, launch and resupply weights, and space occupied, and developmental factors of reliability and reparability. For each subsystem one can identify theoretical power, etc., requirements, and current realistic requirements, and potential realistic subsystem requirements. Compared are reverse osmosis, vapor-compression-distillation, and TIMES processes, in terms of power, weight, and space requirements. Discussions of electrodialysis and CELSS are included as components of water treatment systems.