  The consistent description of unstable particles, renormalons, or other
Schwinger--Dyson-type of solutions within the framework of perturbative gauge
field theories necessitates the definition and resummation of off-shell Green's
functions, which must respect several crucial physical requirements. A
formalism is presented for resummation of off-shell two-point correlation
functions, which is mainly based on arguments of analyticity, unitarity, gauge
invariance and renormalizability. The analytic results obtained with various
methods, including the background field gauges and the pinch technique are
confronted with the physical requirements imposed; to one-loop order the pinch
technique approach satisfies all of them. Using renormalization group
arguments, we discuss issues of uniqueness of the resummation procedure related
to the latter method.
