The quarks of quark models cannot be identified with the quarks of the QCD
Lagrangian. We review the restrictions that gauge field theories place on a
ny description of physical (colour) charges. A method to construct charged
particles is presented. The solutions are applied to a variety of applicati
ons. Their Green's functions are shown to be free of infra-red divergences
to all orders in perturbation theory. The interquark potential is analysed
and it is shown that the interaction responsible for anti-screening results
from the force between two separately gauge invariant constituent quarks.
A fundamental limit on the applicability of quark models is identified.