Galaxy clusters represent the last step of the formation of large scale structures in the Universe. They are both useful cosmological probes and unique astrophysical laboratories. The clusters grow by accretion of surrounding structures and from the merging of subclusters, in very energetic events, eventually forming a diffuse gas phase made of a hot thermal component, but also leading to particle acceleration up to very high energies.
After introducing the role of clusters in our understanding of the assembly of matter in the Universe, I will discuss how we can study their cosmic ray content, in particular at gamma ray energies. The status of the search for the diffuse gamma ray emission from galaxy clusters will be presented, highlighting recent results from the Fermi-LAT satellite. I will also discuss prospects for probing such signal using the Cherenkov Telescope Array observatory.