  Wick's theorem provides a connection between time ordered products of bosonic
or fermionic fields, and their normal ordered counterparts. We consider a
generic pair of operator orderings and we prove, by induction, the theorem that
relates them. We name this the General Wick's Theorem (GWT) because it carries
Wick's theorem as special instance, when one applies the GWT to time and normal
orderings. We establish the GWT both for bosonic and fermionic operators, i.e.
operators that satisfy c-number commutation and anticommutation relations
respectively. We remarkably show that the GWT is the same, independently from
the type of operator involved. By means of a few examples, we show how the GWT
helps treating demanding problems by reducing the amount of calculations
required.
