  We present high-resolution mass reconstructions for five massive
cluster-lenses spanning a redshift range from z = 0.18 - 0.57 utilizing
archival Hubble Space Telescope data and applying galaxy-galaxy lensing
techniques. These detailed mass models were obtained by combining constraints
from the observed strong and weak lensing regimes. Quantifying the local weak
distortions in the shear maps in terms of perturbations induced by the presence
of galaxy halos around individual bright early-type cluster member galaxies, we
estimate the fraction of mass in the central regions of these clusters that can
be associated with small scale mass clumps. This technique enables us to
directly map the substructure in the mass range 10^{11} - 10^{12.5} solar
masses which we associate with galaxy-scale sub-halos. The determination of the
mass spectrum of substructure in the inner regions of these clusters is
presented. Constraints are thereby obtained on the masses, mass-to-light ratios
and truncation radii for these sub-halos. We find that the fraction of total
cluster mass associated with individual sub-halos within the inner 0.5h^{-1} -
0.8h^{-1} Mpc of these clusters ranges from 10 - 20%. Our results have
important implications for the survival and evolution of substructure in high
density cluster cores and are consistent with the theoretical picture of tidal
stripping of galaxy-scale halos in high-density cluster environments as
expected in hierarchical Cold Dark Matter dominated structure formation
scenarios.
