  This paper retraces the 24 years starting with the appearance of the symbol
"$\nu_{\tau}$" in 1977, until the observation of tau neutrino interactions with
matter in 2000. The fact that the neutral particle present in tau decays was a
neutrino was demonstrated by 1979; its existence as the third neutrino
$\nu_{\tau}$, iso-spin partner of the tau lepton, was definitely established in
1981-1986; it was demonstrated that the number of light active neutrinos is
closed with the known ones ($\nu_e, \nu_{\mu},\nu_{\tau}$) in 1989; before 2000
the $\nu_{\tau}$ properties had been precisely determined in $e^+e^-$ and
$p\bar{p}$ collider experiments.
