  Using gluon and ghost propagators obtained from Schwinger-Dyson equations
(SDEs), we construct the non-perturbative effective charge of QCD. We employ
two different definitions, which, despite their distinct field-theoretic
origin, give rise to qualitative comparable results, by virtue of a crucial
non-perturbative identity. Most importantly, the QCD charge obtained with
either definition freezes in the deep infrared, in agreement with theoretical
and phenomenological expectations. The various theoretical ingredients
necessary for this construction are reviewed in detail, and some conceptual
subtleties are briefly discussed.
