  We derive an expression for the Casimir force between slabs with arbitrary
dielectric properties characterized by their reflection coefficients. The
formalism presented here is applicable to media with a local or a non-local
dielectric response, an infinite or a finite width, inhomogeneous dissipative,
etc. Our results reduce to the Lifshitz formula for the force between
semi-infinite dielectric slabs by replacing the reflection coefficients by the
Fresnel amplitudes.
