Electrodeionisation is a process intended to produce very high quality demineralised water for the energy, pharmaceuticals or electronics sectors. It is a technology which combines reverse osmosis and ion exchange technologies. It is presented as modules made up of cationic and anionic resins, mixed with each other and placed between two selective reverse osmosis membranes, one cationic and the other anionic. The ions to be eliminated are captured by the resins then transferred to the selective membranes via an electric current generated by a set of electrodes. Electrodeionisation, combined with pretreatment by reverse osmosis, enables very high resistance to be achieved for the demineralised water > 18 MΩ at low cost and without using chemicals.