This publication provides a simple explanation of how various processes convert sea or brackish water to fresh water. Included are descriptions of the membrane processes (reverse osmosis, electrodialysis, transport depletion, and piezodialysis); the distillation processes (multistage flash distillation, vertical tube distillation, multieffect multistage distillation, vapor compression distillation, and solar humidification); the crystalization processes (vacuum freeze-vapor compression, secondary refrigerant freezing, eutectic freezing, and hydrate formation); and a chemical process (ion exchange). (BB)