Clusters and groups of galaxies are now recognized to be fundamental constituents of the Universe. According to modern theory, most of the material in the Universe that has collapsed into coherent structures lies in groups of galaxies, while clusters are the only entities in the Universe that should be representative of the Universe as a whole. Thus the study of groups and clusters of galaxies is of tremendous importance. One of the most surprising distinguishing characteristics of these, the largest and most massive objects in the Universe, is that they are filled with hot X-ray-emitting gas whose mass exceeds (often by factors of 3–7) the mass of the galaxies in the groups and clusters. This gas is enriched in heavy elements (e.g., oxygen and iron) and is thus the repository of most of the metals in the Universe.