Clues to Radial Migration from the Properties of Outer Disks

The outer disks of galaxies present a unique laboratory for studying the process of disk formation. A considerable fraction of observed disks exhibit a break in their surface brightness profiles. The ubiquity of these features points to a crucial aspect of disk formation which must be explained. Recent theoretical work suggests that such breaks are related to significant amounts of radial migration. We discuss the current observational evidence which supports this picture.