our understanding of processes underlying non-thermal activity in clusters [@Ajello:2008rd].

Gamma rays, covering the highest end of the electro-magnetic spectrum, can potentially provide essential information on high-energy phenomena in groups and clusters of galaxies. So far, these objects have not been detected in gamma-rays, and data from the EGRET telescope on board the Compton Gamma-Ray Observatory [@egretref] have only produced upper limits [@reimer] (see however [@grdetect]). Statistically conclusive discoveries and a true revolution in our understanding of the highest energy phenomena in galaxy clusters are anticipated with the Fermi Gamma-ray Space Telescope, formerly known as GLAST [@glastref], which was successfully launched on June 11, 2008. The main instrument onboard Fermi, the Large Area Telescope (LAT), represents a remarkable jump in sensitivity compared to its predecessor EGRET [@egretref]. The LAT also extends the EGRET energy range (20 MeV to 10 GeV) to much higher gamma-ray energies, up to about 300 GeV. Aside from probing high and ultra-high energy particle physics processes in astrophysical sources, the detection of gamma rays from clusters would also help to establish the properties of the primary proton population within clusters, and possibly clarify its role in various cluster phenomena [@heclusters]. These include e.g. the question of 