  We use the universal mass accretion history recently reported for simulations
of halo formation in the cold dark matter model (CDM) to analyze the formation
and growth of a single halo. We derive the time-dependent density profile three
different ways, based upon three approximations of successively greater
realism: equilibrium, radial orbits, and a fluid approximation. For the
equilibrium model, the density profile is well-fit by either an NFW or Moore
profile over a limited range of radii and scale factors. For the radial orbit
model, we find profiles which are generally steeper than the NFW profile, with
an inner logarithmic slope approaching -2, consistent with a purely radial
collisionless system. In the fluid approximation, we find good agreement with
the NFW and Moore profiles for radii resolved by N-body simulations (r/r_200 >
0.01), and an evolution of concentration parameter nearly identical to that
found in N-body simulations. The evolving structure of cosmological halos is
therefore best understood as the effect of a time-varying rate of mass infall
on a smoothly distributed, isotropic, collisionless fluid.
