  This paper gives an overview of the beam injection and extraction principles
for accelerators. After a brief general introduction, it explains different
methods of injecting the beam for hadron and lepton machines. It describes
single- and multi-turn hadron injection, charge-exchange H- injection, then
betatron and synchrotron injection for leptons. For extraction, it presents
single- and multi-turn extraction, as well as resonant extraction methods.
Finally, the requirements for linking several accelerators by a transfer line
are presented.
