  The relation between the quark-gluon description of QCD and the hadronic
picture is studied up to order alpha_s. The analysis of the spin-1 correlators
is developed within the large NC framework. Both representations are shown to
be equivalent in the euclidean domain, where the Operator Product Expansion is
valid. By considering different models for the hadronic spectrum at high
energies, one is able to recover the alpha_s running in the correlators, to fix
the rho(770) and a1(1260) couplings, and to produce a prediction for the values
of the condensates. The Operator Product Expansion is improved by the large NC
resonance theory, extending its range of validity. Dispersion relations are
employed in order to study the minkowskian region and some convenient sum
rules, specially sensitive to the resonance structure of QCD, are worked out. A
first experimental estimate of these sum rules allows a cross-check of former
determinations of the QCD parameters and helps to discern and to discard some
of the considered hadronical models. Finally, the truncated resonance theory
and the Minimal Hadronical Approximation arise as a natural approach to the
full resonance theory, not as a model.
