  We consider Wick's Theorem for finite temperature and finite volume systems.
Working at an operator level with a path ordered approach, we show that
contrary to claims in the literature, expectation values of normal ordered
products can be chosen to be zero and that results obtained are independent of
volume. Thus the path integral and operator approaches to finite temperature
and finite volume quantum field theories are indeed seen to be identical. The
conditions under which normal ordered products have simple symmetry properties
are also considered.
