  Cosmological N--body simulations have revealed a remarkable similarity in the
structure of dark matter halos formed in hierarchically clustering universes.
Regardless of halo mass, cosmological parameters, and power spectrum of initial
density fluctuations, the spherically averaged density profiles of dark matter
halos have a universal shape. The logarithmic slope of this profile is
shallower than isothermal near the center, and steepens gently outwards,
becoming steeper than isothermal near the halo virial radius. This profile can
be well approximated by a simple formula with only two free parameters: halo
mass and ``characteristic'' density, e.g., the density at the radius where the
logarithmic slope equals the isothermal value. This characteristic density is
proportional to the mean density of the universe at the time of collapse of
each system, and decreases systematically with increasing halo mass, reflecting
the later collapse of more massive halos. I use these results to examine what
constraints can be derived for Cold Dark Matter models from the rotation curves
of disk galaxies.
