Clusters of galaxies are the largest relaxed structures in the Universe.
Clusters generally form by the gravitational merger of smaller clusters and groups.
Major cluster mergers are the most energetic events in the Universe since the Big Bang. These mergers drive shocks in the intracluster gas. These shocks heat the intracluster gas, and may also accelerate nonthermal relativistic particles. X-ray observations of the thermal effects of merger shocks will be discussed.