O(N_f\alpha^2) Corrections to Muon Decay

The calculation of the $O(N_f{\alpha }^2)$ corrections to muon decay is described. These are the 2-loop diagrams containing a massless fermion loop and they form an important gauge-invariant subclass. It is shown that all such diagrams can be expressed in terms of a universal master integral. We focus on the calculation of box diagrams and in particular on the removal of their infrared divergences.