Gamma rays from clusters of galaxies
  The search for gamma radiation in clusters of galaxies represents a precious tool to investigate the history of these large scale structures. Clusters or sources within them accelerate cosmic rays, as demonstrated by the detection of radio halos, hard X-rays and UV emission, and confine them over cosmological time scales. Nonthermal and thermal phenomena may be closely related and observations of gamma rays may tell us about this link. In this paper we review the physics of cosmic ray acceleration and confinement in clusters of galaxies and the related gamma ray signatures. In particular we describe in some detail the role of cluster mergers for the acceleration of nonthermal particles. The perspectives for gamma ray detection with GLAST and with ground based detectors are also discussed. 
Pasquale Blasi (INAF/Arcetri Astrophys. Obs.)