  Globular cluster (GC) systems have now been studied in galaxies ranging from
dwarfs to giants and spanning the full Hubble sequence of morphological types.
Imaging and spectroscopy with the Hubble Space Telescope and large ground-based
telescopes have together established that most galaxies have bimodal color
distributions that reflect two subpopulations of old GCs: metal-poor and
metal-rich. The characteristics of both subpopulations are correlated with
those of their parent galaxies. We argue that metal-poor GCs formed in low-mass
dark matter halos in the early universe and that their properties reflect
biased galaxy assembly. The metal-rich GCs were born in the subsequent
dissipational buildup of their parent galaxies and their ages and abundances
indicate that most massive early-type galaxies formed the bulk of their stars
at early times. Detailed studies of both subpopulations offer some of the
strongest constraints on hierarchical galaxy formation that can be obtained in
the near-field.
