Quark structure of pseudoscalar mesons

I review to which extent the properties of pseudoscalar mesons can be under stood in terms of the underlying quark (and eventually gluon) structure. Sp ecial emphasis is put on the progress in our understanding of eta-eta' mixi ng. Process-independent mixing parameters are defined, and relations betwee n different bases and conventions are studied. Both, the low-energy descrip tion in the framework of chiral perturbation theory and the high-energy app lication in terms of light-cone wave functions for partonic Fock states, ar e considered. A thorough discussion of theoretical and phenomenological con sequences of the mixing approach will be given. Finally, I will discuss mix ing with other states (pi(0), eta(c),...).