  Inertial motion superradiance, the emission of radiation by an initially
unexcited system moving inertially but superluminally through a medium, has
long been known. Rotational superradiance, the amplification of radiation by a
rotating rigid object, was recognized much later, principally in connection
with black hole radiances. Here we review the principles of inertial motion
superradiance and prove thermodynamically that the Ginzburg--Frank condition
for superradiance coincides with the condition for superradiant amplification
of already existing radiation. Examples we cite include a new type of black
hole superradiance. We correct Zel'dovich's thermodynamic derivation of the
Zel'dovich--Misner condition for rotational superradiance by including the
radiant entropy in the bookkeeping . We work out in full detail the
electrodynamics of a Zel'dovich rotating cylinder, including a general
electrodynamic proof of the Zel'dovich--Misner condition, and explicit
calculations of the superradiant gain for both types of polarization. Contrary
to Zel'dovich's pessimistic conclusion we conclude that, if the cylinder is
surrounded by a dielectric jacket and the whole assembly is placed inside a
rotating cavity, the superradiance is measurable in the laboratory.
