  A model is constructed for the mass and dynamics of M87 and the Virgo
Cluster. Existing surface photometry of the galaxy, mass estimates from X-ray
observations of the hot intracluster gas, and the velocity dispersions of
early-type Virgo galaxies, all are used to constrain the run of dark matter
density over radii to 2 Mpc in the cluster. The ``universal'' halo advocated by
Navarro, Frenk, & White provides an excellent description of the combined data,
as does a Hernquist profile. These models are favored over isothermal spheres,
and their central structure is preferred to density cusps either much stronger
or much weaker than r^{-1}. The galaxies and gas in the cluster trace its total
mass distribution, the galaxies' velocity ellipsoid is close to isotropic, and
the gas temperature follows the virial temperature profile of the dark halo.
The virial radius and mass and the intracluster gas fraction of Virgo are
evaluated.
