  The large samples of tau decays available from CLEO and the four LEP
experiment have resulted in new, precise measurements of the leptonic branching
ratios of the $\tau$. The experimental techniques to obtain these results are
reviewed with special emphasis on the DELPHI measurement. World averages are
found to be Be = (17.81 +/- 0.06) % and Bmu = (17.36 +/- 0.06) %. These results
are consistent with universality in the charged current couplings to a
precision of about 0.25 %. The branching ratio measurements can also be used to
constrain the "low energy parameter" eta. It is shown that the sensitivity to
eta depends on details of the momentum acceptance for muon identification in
the different experiments. Assuming universality in the couplings, the estimate
eta = 0.012 +/- 0.024 is obtained.
