  We review the recent status of the QCD sum rule approach to study the
properties of hadrons in vacuum and in hot or dense matter. Special focus is
laid on the progress made in the evaluation of the QCD condensates, which are
the input of all QCD sum rule calculations, and for which much new information
has become available through high precision lattice QCD calculations, chiral
perturbation theory and experimental measurements. Furthermore, we critically
examine common analysis methods for QCD sum rules and contrast them with
potential alternative strategies. The status of QCD sum rule studies
investigating the modification of hadrons at finite density as well as recent
derivations of exact sum rules applicable to finite temperature spectral
functions, are also reviewed.
