Galaxy clusters are the most massive objects in the universe held together by gravity, containing up to several thousand individual galaxies and huge reservoirs of superheated, X-ray-emitting gas. The mass of this hot gas is typically about five times higher than the total mass of all the galaxies in galaxy clusters. In addition to these visible components, 80% of the mass of galaxy clusters is supplied by dark matter. These cosmic giants are bellwethers not only for the galaxies, stars and black holes within them, but also for the evolution and growth of the universe itself.