  Although eta-eta' mixing is qualitatively well understood as a consequence of
the U(1)_A anomaly in QCD together with a broken SU(3) flavor symmetry, until
recently the values of decay and mixing parameters of the eta and eta' were
only approximately known, e.g. values for the octet-singlet mixing angle
between -20 degrees and -10 degrees could be found in the literature. New
experimental data, especially for the reactions gamma gamma* -> eta, eta' and B
-> eta' K, together with new theoretical results from higher order corrections
in chiral perturbation theory stimulated a phenomenological re-analysis of this
subject, which led to a coherent qualitative and quantitative picture of
eta-eta' mixing and even of eta-eta'-eta_c mixing.
