Formation and evolution of E and S0 galaxies from HST and Keck studies of Z similar to 0.3-1 clusters

We have witnessed a dramatic increase over the last five years in results o n distant galaxies, in large part because of the high resolution imaging ca pability of HST, and the multiobject spectroscopic capability of the Keck t elescopes. Our program to obtain wide-field, multi-color WFPC2 mosaics with HST of intermediate redshift clusters, and spectroscopic membership and hi gh S/N spectroscopy with LRIS on Keck, has provided new insights into the n ature of elliptical and S0 galaxies in the cluster environment over a wide range of densities. In particular, most ellipticals, and a significant frac tion of the S0 population, have large luminosity-weighted ages, suggesting that their stellar populations were formed at redshifts beyond z similar to 2, though the existence of substantial numbers of major mergers in MS 1054 -03 at z = 0.83 suggests that final assembly of such galaxies may not have occurred until much later for a significant fraction of early-type galaxies .