Shocks that occur during cluster mergers accelerate particles to relativistic energies. In the presence of magnetic fields, these particles emit synchrotron radiation and may form so-called radio relics. New observations show that shock acceleration operates on scales much larger than in supernova remnants and that shocks in galaxy clusters are capable of producing extremely energetic cosmic rays. We conducted an observing campaign on a range of radio telescopes that resulted in a sample of more than 30 relics. This led to the discovery of some spectacular sources and the discovery of interesting features in the radio spectra as well as correlations between various properties of relics. The correlations are consistent with the scenario that relics trace merger shock waves. However, our observations also challenge models of shock acceleration and magnetogenesis in galaxy clusters. Finally, I will report on first cluster observations with LOFAR.