  The Casimir force between dissipative metallic mirrors at non zero
temperature has recently given rise to contradictory claims which have raised
doubts about the theoretical expression of the force. In order to contribute to
the resolution of this difficulty, we come back to the derivation of the force
from basic principles of the quantum theory of lossy optical cavities. We
obtain an expression which is valid for arbitrary mirrors, including
dissipative ones, characterized by frequency dependent reflection amplitudes.
