  Within our present knowledge, the tau is the heaviest lepton and the only one
decaying into hadrons, a fact that makes it the source of a very rich
phenomenology. It represents the third family of leptons in the Standard Model,
a feature that helps its classification but whose real meaning is not asserted
yet. The tau lepton provides: i) a clean and unique environment to study both
the hadronization of QCD currents, in an energy region populated by resonances,
and the phenomenological determination of relevant parameters of the Model; ii)
together with the muon, they have a very constrained flavour dynamics (in the
absence of neutrino masses) due to an accidental global symmetry of the
Standard Model. In consequence, the tau lepton brings an excellent benchmark
for the study of QCD at low energies and, at the same time, for the search of
new physics.
