  We investigate the mass profile of LambdaCDM halos using a suite of numerical
simulations spanning five decades in halo mass, from dwarf galaxies to rich
galaxy clusters. Our analysis confirms the proposal of Navarro, Frenk & White
(NFW) that the shape of LambdaCDM halo mass profiles differs strongly from a
power law and depends little on mass. The logarithmic slope of the
spherically-averaged density profile, as measured by beta=-dln(rho)/dln(r),
decreases monotonically towards the center and becomes shallower than
isothermal (beta<2) inside a characteristic radius, r_{-2}. Although the
fitting formula proposed by NFW provides a reasonably good approximation to the
density and circular velocity profiles of individual halos, systematic
deviations from the best NFW fits are also noticeable. Inside r_{-2}, the
profile of simulated halos gets shallower with radius more gradually than
predicted and, as a result, NFW fits tend to underestimate the dark matter
density in these regions. This discrepancy has been interpreted as indicating a
steeply divergent cusp, but our results suggest a different interpretation. We
use the density and enclosed mass at our innermost resolved radii to place
strong constraints on beta_{0}: density cusps as steep as r^{-1.5} are
inconsistent with most of our simulations, although beta_{0}=1 is still
consistent with our data. Our density profiles show no sign of converging to a
well-defined asymptotic inner power law. We propose a simple formula that
reproduces the radial dependence of the slope better than the NFW profile, and
so may minimize errors when extrapolating our results inward to radii not yet
reliably probed by numerical simulations.
