  We examine the dynamics of the stars and globular clusters in the nearby
giant galaxy M87 and constrain the mass distribution, using all the available
data over a large range of radii, including higher-order moments of the stellar
line-of-sight velocity distributions and the discrete velocities of over two
hundred globular clusters. We introduce an extension of spherical orbit
modeling methods that makes full use of all the information in the data, and
provides very robust constraints on the mass models. We conclusively rule out a
constant mass-to-light ratio model, and infer that the radial density profile
of the galaxy's dark halo falls off more slowly than 1/r^2, suggesting that the
potential of the Virgo Cluster is already dominant at r ~ 300" ~ 20 kpc.
