Interstellar Grain Alignment - Observational Status
  Interstellar polarization in the optical/infrared has long been known to be
due to asymmetrical dust grains aligned with the magnetic field and can
potentially provide a resource effective way to probe both the topology and
strength of the magnetic field. However, to do so with confidence, the physics
and variability of the alignment mechanisms must be quantitatively understood.
The last 15 years has seen major advancements in both the theoretical and
observational understanding of this problem. I here review the current state of
the observational constraints on the grain alignment physics. While none of the
three classes of proposed grain alignment theories: mechanical, paramagnetic
relaxation and radiative alignment torque, can be viewed as having been
empirically confirmed, the first two have failed some critical observational
tests, whereas the latter has recently been given specific observational
support and must now be viewed as the leading candidate.
