  We consider the definition of the Boulware and Hartle-Hawking states for
quantum fields on black hole space-times. The properties of these states on a
Schwarzschild black hole have been understood for many years, but neither of
these states has a direct analogue on a Kerr black hole. We show how
superradiant modes play an important role in the definition of quantum states
on Kerr. Superradiance is also present on static black hole space-times, in
particular for a charged scalar field on a Reissner-Nordstrom black hole. We
explore whether analogues of the Boulware and Hartle-Hawking states exist in
this situation.
