A description is presented of the development of H- and D- sources for neutral beam heating on the next generation Tokamaks such as NET or ITER. The two basic types of such ion sources are discussed, where either the ion is formed in the plasma by dissociative attachment or on the surface of a negatively biased electron of low work function. Also described are the acceleration techniques to form high energy and power beams needed for neutral beam injectors. Finally, a short discussion is presented of the future development of these sources.