  Rich clusters of galaxies are the most massive virialized systems known. Even
though they contain only a small fraction of all galaxies, rich clusters
provide a powerful tool for the study of galaxy formation, dark matter,
large-scale structure, and cosmology. Superclusters, the largest known systems
of galaxies, extend to (approx) 100h^{-1} Mpc in size and highlight the
large-scale structure of the universe. This large-scale structure reflects
initial conditions in the early universe and places strong contraints on models
of galaxy formation and on cosmology. Some of the questions that can be
addressed with clusters and superclusters of galaxies include: How did galaxies
and larger structures form and evolve? What is the amount, composition, and
distribution of matter in clusters and larger structures? How does the cluster
mass density relate to the matter density in the universe? What constraints can
the cluster and supercluster data place on cosmology? I will discuss in these
lectures some of the properties of clusters and superclusters of galaxies that
can be used to investigate these topics.
