The pole mass in perturbative QCD

We review quark mass definitions in QCD, defending the identification of pole with constituent masses on the one hand and of invariant mass flavour ratios with current mass flavour ratios on the other. We prove that the pole mass is gauge-parameter independent and infrared finite at the two-loop level. We compute the two-loop mass anomalous dimension in the MS renormalization scheme. The renormalization group allows one to connect constituent with current mass ratios. For heavy flavours we predict the invariant masses.