  We briefly review the existing database of supernova spectropolarimetry,
concentrating on recent data and on results from our group's research.
Spectropolarimetry provides the only direct known probe of early-time supernova
geometry. To obtain reliable conclusions, however, it is very important to
correctly account for interstellar polarization. We find that Type IIn
supernovae (SNe IIn) tend to be highly polarized, perhaps in part because of
the interaction of the ejecta with an asymmetric circumstellar medium. In
contrast, SNe II-P are not polarized much, at least shortly after the
explosion. At later times, however, there is evidence for increasing
polarization, as one views deeper into the expanding ejecta. Moreover,
core-collapse SNe that have lost part (SN IIb) or all (SN Ib) of their hydrogen
(or even helium; SN Ic) layers prior to the explosion tend to show substantial
polarization; thus, the deeper we probe into core-collapse events, the greater
the asphericity. There is now conclusive evidence that at least some SNe Ia are
intrinsically polarized, although only by a small amount. Finally, SN
spectropolarimetry provides the opportunity to study the fundamental properties
of the interstellar dust in external galaxies. For example, we have found
evidence for extremely high polarization efficiency for the dust along the
line-of-sight to SN 1999gi in NGC 3184.
