A review of Rb at LEP (

Summary. -- In the Standard Model, the ratio Rb of the Z partial width into bb to its total width into hadrons is sensitive to the mass of the top quark but largely independent of the Higgs mass and the strong-coupling constant. A measurement with a relative precision below one percent would be a landmark test of the predictive powers of the Standard Model. The four LEP Collaborations have presented a series of increasingly precise analyses, using lepton tagging, event shape tagging and lifetime tagging, improving the methods as the data sets have grown in size. The current world average of 0.2205 +- 0.0016, dominated by results from lifetime tagging, is about three standard deviations above the Standard Model value, assuming a top quark mass around 175 GeV/c e. Further improvements in precision are expected.