  N-body simulations find a universal structure for the halos which result from
the nonlinear growth of Gaussian-random-noise density fluctuations in the CDM
universe. This talk summarized our attempts to derive and explain this
universal structure by analytical approximation and simplified models. As an
example, we show here that a 1D spherical infall model involving a fluid
approximation derived from the Boltzmann equation can explain not only the halo
density profile but its phase-space density profile, as well.
