Phenomenological Evidence for the Gluon Content of $\eta$ and $\eta'$

We discuss the effect of $\eta$--$\eta'$ mixing and of axial current anomalies on various decay processes. We describe the 2$\gamma$ decays of $\eta$ and $\eta'$, the radiative decays of low-lying vector mesons as well as the decays $\eta\to 3\pi$, $J/\psi\to\eta\gamma$ and $J/\psi\to\eta'\gamma$ in one unified framework, in which they are related to the electromagnetic and strong axial current anomalies, respectively. We also discuss briefly the enhancement of $\eta'$ in $D_s$ decays and suggest a possible relation to gluon-mediated processes. The importance of the decays of the glueball candidates $f_0(1500)$ and $ f_0(1590)$ into $\eta \eta'$ channels is also stressed.