  We use zeta function techniques to give a finite definition for the Casimir
energy of an arbitrary ultrastatic spacetime with or without boundaries. We
find that the Casimir energy is intimately related to, but not identical to,
the one-loop effective energy. We show that in general the Casimir energy
depends on a normalization scale. This phenomenon has relevance to applications
of the Casimir energy in bag models of QCD.
  Within the framework of Kaluza-Klein theories we discuss the one-loop
corrections to the induced cosmological and Newton constants in terms of a
Casimir like effect. We can calculate the dependence of these constants on the
radius of the compact dimensions, without having to resort to detailed
calculations.
