  Radio-frequency linear accelerators are used as injectors for synchrotrons
and as stand-alone accelerators for the production of intense particle beams,
thanks to their ability to accelerate high beam currents at high repetition
rates. This lecture introduces their main features, reviewing the different
types of accelerating structures used in linacs and presenting the main
characteristics of linac beam dynamics. Building on these bases, the
architecture of modern proton linear accelerators is presented with a
particular emphasis on high-energy and high-beam-power applications.
