In Capacitive Deionization (CDI), charged ionic species are removed from aqueous solutions. The ions are adsorbed onto the surface of a pair of electrically charged electrodes, usually composed of highly porous carbon materials, upon applying an electrical voltage difference. Upon charging the electrodes with a voltage difference of typically 1-1.4 V, the salt ions present in the feed migrate to the electrode of opposite charge, cations to the cathode and anions to the anode, and form electrical double layers (EDLs) along the pore surface. Thus, the water flowing through the CDI cell is partially desalinated. In a discharging step, where either the applied voltage is shorted or the polarity reversed, the salt ions are released in a brine stream. The system architecture can be in flow-by or flow-through mode with the feed either streaming past the electrodes in parallel direction or streaming vertically through the electrode. New developments propose floating electrodes suspended in the aqueous solution that enable continuous operation. Various porous carbon materials have been suggested as electrode material, such as carbon aerogels, activated carbon and carbon nanotubes. An important factor is the sorption performance. The technology has been previously applied to brackish and seawater desalination, wastewater remediation and water softening, but has proven to be highly effective for solutions with low molar concentration such as brackish water. CDI does not require high pressure or temperature, as in membrane or thermal desalination, making the technology more energy efficient in comparison. It also has a higher accuracy in removing only particular salts and ions that enables the recovery of valuable compounds such as lithium among others. However, problems may arise in the regeneration phase as during reversed-polarity, repelled ions might be attracted to the oppositely charged electrode and by electrical shorting the only driving force is diffusion, which is slow and inefficient. The special applicability to brackish water offers a great potential for development, as the demand for desalination of brackish water is increasing due to salt intrusions into the groundwater in many regions worldwide. Nonetheless, many basic settings have not been uniquely defined until now and have to be further optimized.