  We review the phenomenological and theoretical evidences for dynamical gluon
mass generation and the main features of the glueball spectrum in (pure gauge)
Yang-Mills theories. The mixing between glueball and conventional $\bar q q$
states in $f_0$ scalar mesons is discussed. For pseudoscalar mesons, the
inclusion of the glue field in an effective low energy theory is presented
leading to a third isoscalar $\eta''$ partner of the $\eta$ and $\eta'$.
Branching ratios for processes involving $\eta''$ are given and, when
available, compatible with data for $\eta(1405)$.

High Energy Physics - Phenomenology