  The surface-brightness profiles of galaxy disks fall into three main classes,
based on whether they are simple exponentials (Type I), bend down at large
radii (Type II, "truncations") or bend up at large radii (Type III,
"antitruncations"). Here, we discuss how the frequency of these different
profiles depends on Hubble type, environment, and the presence or absence of
bars; these trends may herald important new tests for disk formation models.
