  Dark matter-dominated cluster-scale halos act as an important cosmological
probe and provide a key testing ground for structure formation theory. Focusing
on their mass profiles, we have carried out (gravity-only) simulations of the
concordance LCDM cosmology, covering a mass range of 2.10^{12}-2.10^{15} solar
mass/h and a redshift range of z=0-2, while satisfying the associated
requirements of resolution and statistical control. When fitting to the
Navarro-Frenk-White profile, our concentration-mass (c-M) relation differs in
normalization and shape in comparison to previous studies that have limited
statistics in the upper end of the mass range. We show that the flattening of
the c-M relation with redshift is naturally expressed if c is viewed as a
function of the peak height parameter, \nu. Unlike the c-M relation, the slope
of the c-\nu relation is effectively constant over the redshift range z=0-2,
while the amplitude varies by ~30% for massive clusters. This relation is,
however, not universal: Using a simulation suite covering the allowed wCDM
parameter space, we show that the c-\nu relation varies by about +/- 20% as
cosmological parameters are varied. At fixed mass, the c(M) distribution is
well-fit by a Gaussian with \sigma_c/c = 0.33, independent of the radius at
which the concentration is defined, the halo dynamical state, and the
underlying cosmology. We compare the LCDM predictions with observations of halo
concentrations from strong lensing, weak lensing, galaxy kinematics, and X-ray
data, finding good agreement for massive clusters (M > 4.10^{14} solar mass/h),
but with some disagreements at lower masses. Because of uncertainty in
observational systematics and modeling of baryonic physics, the significance of
these discrepancies remains unclear.
