M31, M33 and the Milky Way: Similarities and Differences
  The large galaxies in the Local Group, while all disk galaxies, have diverse stellar populations. A better understanding of these differences, and a physical understanding of the causes, requires more detailed study of the older populations. This presents a significant challenge to GAIA but the scientific returns are also significant. 
Rosemary F.G. Wyse (JHU)