  For a system of correlated electrons, the Luttinger-Ward functional provides
a link between static thermodynamic quantities on the one hand and
single-particle excitations on the other. The functional is useful to derive
several general properties of the system as well as for the formulation of
thermodynamically consistent approximations. Its original construction,
however, is perturbative as it is based on the weak-coupling skeleton-diagram
expansion. Here, it is shown that the Luttinger-Ward functional can be derived
within a general functional-integral approach. This alternative and
non-perturbative approach stresses the fact that the Luttinger-Ward functional
is universal for a large class of models.
