  Thermal effects on the creation of particles under the influence of
time-dependent boundary conditions are investigated. The dominant temperature
correction to the energy radiated by a moving mirror is derived by means of
response theory. For a resonantly vibrating cavity the thermal effect on the
number of created photons is obtained non-perturbatively. Finite temperatures
can enhance the pure vacuum effect by several orders of magnitude. The
relevance of finite temperature effects for the experimental verification of
the dynamical Casimir effect is addressed.
