  Studies of stellar populations, understood to mean collections of stars with
common spatial, kinematic, chemical, and/or age distributions, have been
reinvigorated during the last decade by the advent of large-area sky surveys
such as SDSS, 2MASS, RAVE, and others. We review recent analyses of these data
that, together with theoretical and modeling advances, are revolutionizing our
understanding of the nature of the Milky Way, and galaxy formation and
evolution in general. The formation of galaxies like the Milky Way was long
thought to be a steady process leading to a smooth distribution of stars.
However, the abundance of substructure in the multi-dimensional space of
various observables, such as position, kinematics, and metallicity, is by now
proven beyond doubt, and demonstrates the importance of mergers in the growth
of galaxies. Unlike smooth models that involve simple components, the new data
reviewed here clearly show many irregular structures, such as the Sagittarius
dwarf tidal stream and the Virgo and Pisces overdensities in the halo, and the
Monoceros stream closer to the Galactic plane. These recent developments have
made it clear that the Milky Way is a complex and dynamical structure, one that
is still being shaped by the merging of neighboring smaller galaxies. We also
briefly discuss the next generation of wide-field sky surveys, such as
SkyMapper, Pan-STARRS, Gaia and LSST, which will improve measurement precision
manyfold, and comprise billions of individual stars. The ultimate goal,
development of a coherent and detailed story of the assembly and evolutionary
history of the Milky Way and other large spirals like it, now appears well
within reach.
