Our present understanding of the strong interactions governing quark
dynamics is based on the QCD lagrangian from which no reliable
information can be extracted except for asymptotic 
-values. In
this region, quark counting rule predictions can be deduced as we have
already briefly discussed. In the remaining 
-regions, accessible to
low-energy 
machines, no valid theoretical predictions are
available and several phenomenological models have been proposed.
Examples are the Vector Meson Dominance (VMD) model (where the
similarities between vector-mesons and photons are exploited) and
constituent quark models with their underlying SU(3) symmetry (where
the hadronic constituents are phenomenologically taken into account).
- or QCD-duality [18] can be invoked to link these two
approaches, but for theoretically more justified points of view one has
to rely on the Chiral Perturbation Theory (ChPT) approach to
pseudoscalar charge radii [15] and [19].