  The hierarchical theory of galaxy formation rests on the idea that smaller
galactic structures merge to form the galaxies that we see today. The past
decade has provided remarkable observational support for this scenario, driven
in part by advances in spectroscopic instrumentation. Multi-object spectroscopy
enabled the discovery of kinematically cold substructures around the Milky Way
and M31 that are likely the debris of disrupting satellites. Improvements in
high-resolution spectroscopy have produced key evidence that the abundance
patterns of the Milky Way halo and its dwarf satellites can be explained by
Galactic chemical evolution models based on hierarchical assembly.
  These breakthroughs have depended almost entirely on observations of nearby
stars in the Milky Way and luminous red giant stars in M31 and Local Group
dwarf satellites. In the next decade, extremely large telescopes will allow
observations far down the luminosity function in the known dwarf galaxies, and
they will enable observations of individual stars far out in the Galactic halo.
The chemical abundance census now available for the Milky Way will become
possible for our nearest neighbor, M31. Velocity dispersion measurements now
available in M31 will become possible for systems beyond the Local Group such
as Sculptor and M81 Group galaxies. Detailed studies of a greater number of
individual stars in a greater number of spiral galaxies and their satellites
will test hierarchical assembly in new ways because dynamical and chemical
evolution models predict different outcomes for halos of different masses in
different environments.
