Neutral beams to heat the plasmas of controlled fusion experiments are generated by neutralizing the charge of positive ions after they have been focused and accelerated. The efficiency of this system decreases with ion velocity and for D+ the system is practical only at ion energies of less than about 150 keV,which is less than the energies expected to be required in the future. However, the use of negative ions, where neutralization is accomplished by removing, rather than by adding electrons, allows much higher ion energies to be efficiently employed. Programs to develop sources of large currents of H− and D− are therefore underway. The goal is to produce practical sources which can supply ampere beams of continuous, impurity-free, negative hydrogen ions in geometries that can be extended to even larger currents. This paper briefly describes the three main methods of producing negative ions and the advantages and disadvantages of each.