  The ground state energy of a boundary quantum field theory is derived in
planar geometry in D+1 dimensional spacetime. It provides a universal
expression for the Casimir energy which exhibits its dependence on the boundary
conditions via the reflection amplitudes of the low energy particle
excitations. We demonstrate the easy and straightforward applicability of the
general expression by analyzing the free scalar field with Robin boundary
condition and by rederiving the most important results available in the
literature for this geometry.
