  Clusters of galaxies, the most massive virialized systems known, provide a
powerful tool for studying the structure, the mass density, and the cosmology
of our universe. Clusters furnish one of the best estimates of the dynamical
mass density parameter on 1 Mpc scale, $\Omega_{dyn}$; the best measure of the
baryon density fraction in the universe; an excellent tracer of the large-scale
structure of the universe; and, most recently, a powerful tracer of the
evolution of structure in the universe and its unique cosmological
implications. I review the above measures and show that they portray a
consistent picture of the universe and place stringent constraints on
cosmology. Each of these independent measures suggests a low-density universe,
$\Omega_{m}$ $\simeq $ 0.3 $\pm $ 0.1, with mass approximately tracing light on
large scales.

Astrophysics