  We investigate the phenomenon of quantum radiation - i.e. the conversion of
(virtual) quantum fluctuations into (real) particles induced by dynamical
external conditions - for an initial thermal equilibrium state. For a
resonantly vibrating cavity a rather strong enhancement of the number of
generated particles (the dynamical Casimir effect) at finite temperatures is
observed. Furthermore we derive the temperature corrections to the energy
radiated by a single moving mirror and an oscillating bubble within a
dielectric medium as well as the number of created particles within the
Friedmann-Robertson-Walker universe. Possible implications and the relevance
for experimental tests are addressed. PACS: 42.50.Lc, 03.70.+k, 11.10.Ef,
11.10.Wx.
