This brief review discusses various aspects of the description of hadrons as bound states of quarks at the simpler example of mesons, which are regarded as composite of a (constituent) quark and antiquark. It is shown how a recently derived relativistic virial theorem may be used in order to cast some light on the interplay between the (semi-)relativistic and (maybe only effective) nonrelativistic treatment of bound states. Furthermore, a new relativistic approach to fermion-antifermion bound states is briefly sketched and applied in order to predict analytically a few selected general features of the meson spectrum. 27 refs.; 2 figs.; 1 tab.