  A systematic study of the $\eta$-$\eta'$ mixing in Large-$N_c$ chiral
perturbation theory is presented with special emphasis on the role of the
next-to-next-to-leading-order contributions in the combined momentum,
quark-mass, and $1/N_c$ expansions. At this order, loop corrections as well as
OZI-rule-violating pieces need to be included. Mixing angles as well as
pseudoscalar decay constants are discussed within this framework. The results
are compared with recent phenomenological approaches.
