Clusters and superclusters of galaxies are the largest objects in the Universe and are the subject of intense observational study at a variety of wavelengths, from radio to X-ray which has provoked much theoretical debate and advanced the understanding of the recent evolution of the large-scale structure the universe. The subject is reviewed in this volume by researchers who lectured at a NATO Advanced Study Institute held in Cambridge in July 1991. Much of the material is presented in a pedagogical manner and should be useful to scientists, astronomers and graduate students interested in extragalactic astronomy.