  The chiral expansion of the low energy processes $\pi^0\to\gamma\gamma$ and
$\eta\to\gamma\gamma$ is reconsidered with particular emphasis on the question
of the evaluation of the two low-energy parameters from ${\cal L}^{WZ}_{(6)}$
which are involved at chiral order six. It is shown how extensive use of
sum-rules and saturation with resonances as well as constraints from asymptotic
QCD effectively determine their values. Predictions for the widths are
presented for both standard and non-standard values of the quark mass ratio
$m_s/{\hat m}$. A precise relation is established between the usual
phenomenological $\eta-\eta'$ mixing parameters and those of the chiral
expansion. The large size of the chiral correction to the $\eta$ decay can be
understood on the basis of a simple counting rule: $O(1/N_c)\sim\ O(m_q)$. It
is shown how this counting rule eventually allows one to include the $\eta'$
into the effective lagrangian in a consistent and systematic way.
