  I review our current knowledge of the mass distribution in clusters, as
obtained from the analysis of the projected phase-space distribution of cluster
galaxies. I discuss the methods of analysis, their relative advantages and
disadvantages, and their reliabilities. I summarize the most recent and
important results on the mass distributions of galaxy systems, from nearby to
medium-distant systems, and from groups to clusters. In particular I consider
how well different proposed models fit the observed cluster mass distribution,
and which are the relative distributions of galaxies, baryons, and dark or
total mass in clusters. I also discuss the current observational evidence for
mass accretion onto galaxy systems, coming mostly from the analysis of the
velocity anisotropy profiles.
