  The consistent description of resonant transition amplitudes within the
framework of perturbative field theories necessitates the definition and
resummation of off-shell Green's functions, which must respect several crucial
physical requirements. In particular, the generalization of the usual
Breit-Wigner formalism in a non-Abelian context constitutes a highly
non-trivial problem, related to the fact that the conventionally defined
Green's functions are unphysical. We briefly review the main field-theoretical
difficulties arising when attempting to use such Green's functions outside the
confines of a fixed order perturbative calculation, and explain how this task
has been successfully accomplished in the framework of the pinch technique.
