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. ; Comment: 10 pages, 2 eps figures, LaTeX2e file. v2: A reference is added, some minor modifications made to clarify the text. v3: 9 pages, 3 eps figures, LaTeX2e file, revtex style. Paper throughly restructured and rewritten. Much more details are given, but essential results and conclusions are unchanged. Version accepted for publication