  Diffuse intracluster light (ICL) has now been observed in nearby and in
intermediate redshift clusters. Individual intracluster stars have been
detected in the Virgo and Coma clusters and the first color-magnitude diagram
and velocity measurements have been obtained. Recent studies show that the ICL
contains of the order of 10% and perhaps up to 30% of the stellar mass in the
cluster, but in the cores of some dense and rich clusters like Coma, the local
ICL fraction can be high as 40%-50%. What can we learn from the ICL about the
formation of galaxy clusters and the evolution of cluster galaxies? How and
when did the ICL form? What is the connection to the central brightest cluster
galaxy? Cosmological N-body and hydrodynamical simulations are beginning to
make predictions for the kinematics and origin of the ICL. The ICL traces the
evolution of baryonic substructures in dense environments and can thus be used
to constrain some aspects of cosmological simulations that are most uncertain,
such as the modeling of star formation and the mass distribution of the
baryonic component in galaxies.
