  In spite of the promise of new techniques for constraining the mass
distribution in galaxy clusters, much remains to be learned from galaxy orbital
velocities. This article reviews the theory of potential estimation in hot
dynamical systems like galaxy clusters. An analysis is presented of the Coma
cluster, based on a sample of about 1500 galaxies with probable membership, of
which 450 have measured velocities. We find no significant evidence for a core
in either the galaxy number densities or the mass distribution.
