Operator product expansion and duality at finite temperature

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.