  The RF systems installed in synchrotrons can be used to change the
longitudinal beam characteristics. 'RF gymnastics' designates manipulations of
the RF parameters aimed at providing such non-trivial changes. Some keep the
number of bunches constant while changing bunch length, energy spread,
emittance, or distance between bunches. Others are used to change the number of
bunches. After recalling the basics of longitudinal beam dynamics in a hadron
synchrotron, this paper deals with the most commonly used gymnastics. Their
principle is described as well as their performance and limitations.
