W Mass and its Uncertainty from Modelling the Hadronic Final State at   LEP
  From 1996 up to 2000 the LEP collider at CERN has operated at center of mass energies above the production threshold for W boson pairs of approximately 160 GeV. The obtained data are used to extract a preliminary W mass value of (80.450 +- 0.039) GeV by direct reconstruction. To a large extent the uncertainty is due to systematic effects especially in the fully hadronic decay channel W+W- --> qq' qq' that suffers most from ambiguities in modelling the hadronic final state. Methods to assess and reduce these uncertainties are the current main concern of the four LEP experiments. 
K. Rabbertz (CERN, Switzerland)