  We study the influence of finite conductivity of metals on the Casimir
effect. We put the emphasis on explicit theoretical evaluations which can help
comparing experimental results with theory. The reduction of the Casimir force
is evaluated for plane metallic plates. The reduction of the Casimir energy in
the same configuration is also calculated. It can be used to infer the
reduction of the force in the plane-sphere geometry through the `proximity
theorem'. Frequency dependent dielectric response functions of the metals are
represented either by the simple plasma model or, more accurately, by using the
optical data known for the metals used in recent experiments, that is Al, Au
and Cu. In the two latter cases, the results obtained here differ significantly
from those published recently.
