The linear accelerator was invented very early in the history of particle accelerators, but it has been one of the latest accelerators to be exploited. This is principally because of the very large quantities of radio‐frequency power required to attain respectable energies in a reasonable distance. Radar developments during World War II resulted in production of the necessary megawatt oscillators or amplifiers, and linear accelerators, both for electrons and positive ions, are now operating in several centers. The electron linear accelerator has been extended to billion‐volt energies, and in the Stanford two‐mile version it will soon set new energy records between 20 and 40 BeV. The proton linear accelerator has had a less spectacular history. The highest energy yet achieved in a proton linac is about 70 MeV (at the University of Minnesota). Smaller proton linacs are in use as injectors for proton synchrotrons, but no machine has been built or is under construction for the range above 100 MeV. This is because synchrocyclotrons for this energy range are much cheaper and have been preferred for this reason, in spite of the fact that the beam from a synchrocyclotron cannot be nearly as intense or as well collimated as the beam from a linear accelerator.