  The most successful cosmological models to date envision structure formation
as a hierarchical process in which gravity is constantly drawing lumps of
matter together to form increasingly larger structures. Clusters of galaxies
currently sit atop this hierarchy as the largest objects that have had time to
collapse under the influence of their own gravity. Thus, their appearance on
the cosmic scene is also relatively recent. Two features of clusters make them
uniquely useful tracers of cosmic evolution. First, clusters are the biggest
things whose masses we can reliably measure because they are the largest
objects to have undergone gravitational relaxation and entered into virial
equilibrium. Mass measurements of nearby clusters can therefore be used to
determine the amount of structure in the universe on scales of 10^14 to 10^15
solar masses, and comparisons of the present-day cluster mass distribution with
the mass distribution at earlier times can be used to measure the rate of
structure formation, placing important constraints on cosmological models.
Second, clusters are essentially ``closed boxes'' that retain all their gaseous
matter, despite the enormous energy input associated with supernovae and active
galactic nuclei, because the gravitational potential wells of clusters are so
deep. The baryonic component of clusters therefore contains a wealth of
information about the processes associated with galaxy formation, including the
efficiency with which baryons are converted into stars and the effects of the
resulting feedback processes on galaxy formation. This article reviews our
theoretical understanding of both the dark-matter component and the baryonic
component of clusters. (Abridged)
