  We calculate the Casimir energy for scalar and gauge fields in interaction
with zero-width mirrors, including quantum effects due to the matter fields
inside the mirrors. We consider models where those fields are either scalar or
fermionic, obtaining general expressions for the energy as a function of the
vacuum field 1PI function. We also study, within the frame of a concrete model,
the role of the dissipation induced by those degrees of freedom, showing that,
after integration of the matter fields, the effective theory for the
electromagnetic field contains modes with complex energies. As for the case of
Lifshitz formula, we show that the formal result obtained by neglecting
dissipation coincides with the correct result that comes from the quantum
fluctuations of both bulk and matter fields.
