A review is given on the quantum correction Δr in the W--Z mass
correlation at the electroweak two-loop level, as derived from the calculation
of the muon lifetime in the Standard Model. Exact results for Δr and
the W-mass prediction including O(α2) corrections with
fermion loops are presented and compared with previous results of a
next-to-leading order expansion in the top-quark mass.Comment: 14 pages, including 4 figures. Presented at the 5th International
Symposium on Radiative Corrections, (RADCOR-2000), Carmel CA, USA, 11-15
September 200