Masses, decays and mixings of gluonia in QCD

This is a review of the estimate of the gluonia masses, decay and mixings from QCD spectral sum rules and low-energy theorems. Some phenomenological maximal gluonium-quarkonium mixing shemes in the scalar sector are presented. This talk is a compact version of the work in Ref.[1] (hep-ph/9612457) having the same title.