The determination of the Fermi coupling constant, G_F, is examined in the
light of recently calculated 2-loop QED corrections and planned experiments tomeasure the muon lifetime to a level below 1ppm. The methods used in the
calculation of the QED corrections are described in detail. Sources of thedominant theoretical and experimental uncertainties are identified. Finally the
incorporation of G_F into analyses using the full electroweak Standard Model is
discussed.