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.