  It has become clear during the last decades that the interaction between the
supernova ejecta and the circumstellar medium is playing a major role both for
the observational properties of the supernova and for understanding the
evolution of the progenitor star leading up to the explosion. In addition, it
provides an opportunity to understand the shock physics connected to both
thermal and non-thermal processes, including relativistic particle
acceleration, radiation processes and the hydrodynamics of shock waves. This
chapter has an emphasis on the information we can get from radio and X-ray
observations, but also their connection to observations in the optical and
ultraviolet. We first review the different physical processes involved in
circumstellar interaction, including hydrodynamics, thermal X-ray emission,
acceleration of relativistic particles and non-emission processes in the radio
and X-ray ranges. Finally, we discuss applications of these to different types
of supernovae.
