  We present new results from our comprehensive comparative survey of two
massive, intermediate redshift galaxy clusters, Cl0024+17 (z=0.39) and
MS0451-03 (z=0.54). We identify and study several key classes of `transition
objects' whose stellar populations or dynamical states indicate a recent or
ongoing change in morphology and star formation rate. For the first time, we
have been able to conclusively identify spiral galaxies in the process of
transforming into S0 galaxies. This has been accomplished by locating both
spirals whose star formation is being quenched as well as their eventual
successors, the recently created S0s. Differences between the two clusters in
both the timescales and spatial location of this conversion process allow us to
evaluate the relative importance of several proposed physical mechanisms that
could be responsible for the transformation. Combined with other diagnostics
that are sensitive to either ICM-driven galaxy evolution or galaxy-galaxy
interactions, we describe a self-consistent picture of galaxy evolution in
clusters. We find that spiral galaxies within infalling groups have already
begun a slow process of conversion into S0s primarily via gentle galaxy-galaxy
interactions. The fates of spirals upon reaching the core of the cluster depend
heavily on the cluster ICM, with rapid conversion of all remaining spirals into
S0s via ram-pressure stripping in clusters where the ICM is dense. In the
presence of a less-dense ICM, the conversion continues at a slower pace, with
galaxy-galaxy interactions continuing to play a role along with `starvation' by
the ICM. We conclude that the buildup of the local S0 population through the
transformation of spiral galaxies is a heterogeneous process that nevertheless
proceeds robustly across a variety of different environments.
