  The operator product expansion of current correlators at short distances, and
the notion of QCD-hadron duality are the cornerstone of QCD sum rules. The
extension of this programme to $T \neq 0$ is discussed, together with
applications to hot hadronic propagators. Indications are that the hadronic
spectrum suffers a substantial rearrangement with increasing temperature, and
hint on the existence of a quark deconfining phase transition. Phenomenological
order parameters to characterize this phase transition are discussed.
