  We investigate the internal structure of cold dark matter halos using high
resolution N-body simulations. As the mass and force resolution are increased,
halo density profiles become steeper, asymptoting to a slope of $\sim -4/3$ in
the central regions and may not have converged to a unique result. At our
highest resolution we have nearly 3 million particles within the virial radius,
$R_{200}$, and force softening that is $\sim 0.2% R_{200}$. This resolution has
also allowed us to resolve a large part of the overmerging problem - we find
over 1000 surviving dark halos orbiting within a single cluster potential.
These data have given us unprecedented insights into the dynamics and structure
of ``halos within halos'', allowing us for the first time, to compare the
distribution of dark matter with observations of galaxies in clusters.
