  Navarro, Frenk, and White have suggested that the density profiles of
simulated dark matter halos have a ``universal'' shape so that a given halo can
be characterized by a single free parameter which fixes its mass. In this
paper, we revisit the spherical infall model in the hope of recognizing in
detail the existence and origin of any such universality. A system of particles
is followed from linear perturbation, through first shell crossing, then
through an accretion or infall phase, and finally to virialization. During the
accretion phase, the system relaxes through a combination of phase mixing,
phase space instability, and moderate violent relation. It is driven quickly,
by the flow of mass through its surface, toward self-similar evolution. The
self-similar solution plays its usual role of intermediate attractor and can be
recognized from a virial-type theorem in scaled variables and from our
numerical simulations. The transition to final equilibrium state once infall
has ceased is relatively gentle, an observation which leads to an approximate
form for the distribution function of the final system. The infall phase fixes
the density profile in intermediate regions of the halo to be close to r^{-2}.
We make contact with the standard hierarchical clustering scenario and explain
how modifications of the self-similar infall model might lead to density
profiles in agreement with those found in numerical simulations.
