  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 any
description of physical (colour) charges. A method to construct charged
particles is presented. The solutions are applied to a variety of applications.
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.
