  We show that exact results are obtained for the calculation of Casimir forces
between arbitrary materials using the concept of surface impedances, obtaining
in a trivial way the force in the limit of perfect conductors and also Lifshitz
formula in the limit of semi-infinite media. As an example we present a full
and rigorous calculation of the Casimir force between two metallic half-spaces
described by a hydrodynamic nonlocal dielectric response.
