Nonperturbative effects in τ decay

It has been suggested that the lifetime of the τ lepton can be calculated using perturbative QCD, with sufficient accuracy that comparison with the measured lifetime can be used to determine the QCD scale parameter Λ. Contrary to this suggestion, nonperturbative effects are shown here to introduce uncertainties in the calculated lifetime which are so large as to make the determination of Λ unreliable. These nonperturbative effects are associated with hadronic thresholds and resonances, and are much larger than previous estimates based on QCD sum rules. Independent experimental tests, which make use of the hadronic mass spectrum in τ decay, are proposed.