  We review the properties of globular clusters which make them useful for
studying the Galactic halo, Galactic chemical evolution, and the early stages
of the formation of the Milky Way. We review the evidence that GCs have a
chemical inventory similar to those of halo field stars. We discuss the
abundance ratios for dSph galaxies and show that it is possible to have formed
at least part the Galactic halo field stellar population by dissolving globular
clusters and/or accreting dSph galaxies but only if this occurred at an early
stage in the formation of the Galaxy. We review the constraints on halo
formation timescales deduced from the low Mg isotopic ratios in metal-poor halo
field dwarfs which indicate that AGB stars did not have time to contribute
significantly, while M71 contains two populations, one without and also one
with a substantial AGB contribution. We review the limited evidence for GCs
with a second population showing additional contributions from SNII, currently
confined to Omega Cen, M54, and M22, all of which may have been the nuclei or
central regions of accreted galaxies. We check our own data for additional such
GCs, and find preliminary indications that NGC 2419, a massive GC far in the
outer Galactic halo, may also belong to this group.
