Capacitive deionization (CDI) is an emerging and promising new technology for removal of ionic as well as polarizable species from water. It is an alternative to membrane-based technologies, having low operational cost, enhanced energy efficiency, and less water rejection. The technology works on the principle of electroadsorption of ions at the surface of electrically charged electrodes, generally made of porous carbon materials. The concept of CDI date back to the 1960s but the community of CDI has grown exponentially in last two decade. The chapter deals with the history of CDI, different patents which revolutionized the technology and the theory of electroadsorption. Different advancements in the technology such as introduction of different cell architectures, parameters responsible for efficient CDI cells, development of different novel electrode materials such as carbon aerogel (CA), carbon cloth, carbon nanotubes (CNTs), graphene and its composites, carbon fibers, and mexenes, and commercialization of CDI products have been discussed in detail. The chapter also gives a brief outlook on the current status and future development of this technology. © 2019 Elsevier Inc. All rights reserved.