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The positive or negative ion sources which form the primary components of neutral beam injection systems used in controlled nuclear fusion using magnetic confinement have to meet simultaneously several demanding requirements. This paper describes the underlying physics of modern positive ion sources, which provide the required high proton fraction (>90%) and high current density (≈2 kA/m2) at a low source pressure (0.4 Pa) with a high electrical efficiency and uniformity across the accelerator grids. The development of negative ion sources, which are required if high energy neutral beams are to be produced, is described, and the present understanding of the physics of negative ion production in sources is explained. The paper reports that negative ion sources have achieved many of the parameters required of sources for the neutral beam injectors of future fusion devices and reactors, >200 A/m2 of D- at low source pressure, <0.3 Pa, with a low co-extracted electron content. The development needed to meet all the requirements of future systems is briefly discussed.