  This compact review about gluonium focuses on a slate of theoretical efforts;
among the many standing works, I have selected several that are meant to assist
in the identification, among ordinary mesons, of the few Yang-Mills glueball
configurations that populate the energy region below 3 GeV. This includes
$J/\psi$ radiative and vector-meson decays, studies of scalar meson mixing, of
high-energy cross sections via the Pomeron and the odderon, glueball decays,
etc. The weight of accumulated evidence seems to support the $f_0(1710)$ as
having a large (and the largest) glueball component among the scalars, although
no single observable by itself is conclusive. Further tests would be welcome,
such as exclusive $f_J$ production at asymptotically high $s$ and $t$. No clear
experimental candidates for the pseudoscalar or tensor glueball stand out yet,
and continuing investigations trying to sort them out will certainly teach us
much more about mesons.
