  We review the diagrammatic, conserving theory for quantum impurities with
strong on-site repulsion. The method is based on auxiliary particle technique,
where Wick's theorem is valid, which opens up the possibility for
generalizations to more complicated situations. An analysis in terms of the
perturbative renormalization group (RG) shows that on the level of the
Conserving T--matrix Approximation the theory correctly describes the RG flow,
including the non-scaling of potential scattering terms and the correct Kondo
temperature.
