  Rich clusters of galaxies, the largest virialized systems known, provide a
powerful tool for the study of cosmology. Some of the fundamental questions
that can be addressed with clusters of galaxies include: how did galaxies and
large-scale structure form and evolve? What is the amount, composition and
distribution of matter in the universe? I review some of the studies utilizing
clusters of galaxies to investigare, among others:
  - The dark matter on clusters scale and the mean mass-density of the
universe;
  - The large-scale structure of the universe;
  - The peculiar velocity field on large scales;
  - The mass-function of groups and clusters of galaxies;
  - The constraints placed on specific cosmological models using the cluster
data.
