Cosmology with Clusters of Galaxies

Rich clusters of galaxies, the largest virialized systems known, place some of the most powerful constraints on cosmology. I discuss below the use of clusters of galaxies in addressing two fundamental questions: What is the mass-density of the universe? and how is the mass distributed? I show that several independent methods utilizing clusters of galaxies - cluster dynamics and mass-to-light ratio, baryon fractions in clusters, and cluster evolution -all indicate the same robust result: the mass-density of the universe is low, Ω_m ≃ 0.2, and the mass approximately traces light on large scales.