Capacitive deionization (CDI) has attracted the interest of the Community investigating water treatment technologies since the mid-1960s. The technology is based oil the recognition that high-surface-area electrodes, when electrically charged, can quantitatively adsorb ionic components from water, thereby resulting in desalination. The article reviews the theoretical and technological background of CDI, the history of its development, and past and Present attempts towards scaling Up and commercialization. It also provides a critical review of the advantages and limitations of the technology. In addition, further research approaches necessary to render CDI a mature and competitive technology for the treatment of brackish and Surface waters are outlined.