  The current understanding of radiation tollerance of Silicon Photomultipliers
(SiPMs) is reviewd. Radiation damage in silicon sensors is briefly introduced,
surface and bulk effects are separately addressed. Results on the operation of
irradiated SiPMs with X-ray, gamma, electron, proton and neutron sources are
presented. The most critical effect of radiation on SiPMs is the increase of
dark count rate, which makes it impossible to resolve signals generated by a
single photon from the noise. Methods to characterize irradiated SiPMs after
their single photo-electron resolution is lost are discussed. Due to the
important similarity in the operation below the breakdown voltage, also older
studies on radiadion damage of avalange photodiodes (APD) are reviewed.
Finally, ideas are presented on how to approach the development of radiation
hard SiPMs in the future.
