  This review presents the results of ongoing studies of the formation
histories of metal-poor and metal-rich globular clusters and their host
galaxies. I first discuss the strong observational evidence that the globular
cluster systems of most elliptical galaxies have bimodal metallicity
distributions. I then focus on new results for metal-poor and metal-rich
globular cluster systems. Metal-poor globular clusters are often associated
with early structure formation, and I review new constraints on their formation
epoch based on the ``bias'' of the number of metal-poor clusters with host
galaxy mass. For metal-rich globular clusters, I discuss new results from
ongoing optical to near-infrared photometric studies which both confirm an
intermediate-age population in NGC 4365 and generally reveal a variety of
formation histories for now quiescent ellipticals.
