Clusters of galaxies are gravitationally bound configurations containing typically hundreds of galaxies in a region about 10 cm in size. They range from irregular clusters, with strong subclustering, no strong central concentration, and mainly spiral galaxies, to regular clusters, with smooth and centrally condensed galaxy distributions containing few spiral galaxies. Observations of these clusters of galaxies show that they are bright x-ray sources, with luminosities of 1043 45 ergs/sec. It is now established that the emission mechanism is thermal bremsstrahlung from hot (-10 K}, low-density (-10 atoms/cm ) gas. This intracluster gas has a distribution similar to that of the galaxies in the cluster and fills the space between the galaxies. Remarkably, the total mass of hot gas in a typical cluster is similar to the total mass in all the stars in all the galaxies in the cluster. The x-ray spectra of clusters show strong x-ray line emission from iron and other heavy elements; this indicates that a significant portion of the intracluster gas must have been ejected from galaxies in the cluster. Recent x-ray observations from the Einstein x-ray satellite show that intracluster gas is cooling in some clusters and being accreted ontc large, central galaxies. X-ray images from Einstein also suggest that the morphology of the gas mirrors the dynamical state of the cluster. In this paper the x-ray observations of clusters of galaxies are reviewed. Related optical and radio measurements of clusters are also summarized. Theories for the physical state, distribution, origin, and evolution of the intracluster medium are extensively discussed. Finally, the author comments. on the prospects for future x-ray observations.
