Phenomenological evidence for the gluon content of η and η′

We discuss the effect of η-η′ mixing and of axial current anomalies on various decay processes. We describe the 2γ decays of η and η′, the radiative decays of low-lying vector mesons as well as the decays η → 3π, Jψ → ηγ and Jψ → η′γ 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 η′ in Ds decays and suggest a possible relation to gluon-mediated processes. The importance of the decays of the glueball candidates f0(1500) and f0(1590) into ηη′ channels is also stressed.