  The relevance of the data concerning upward--going muons for the solution of
the atmospheric neutrino problem is stressed. In particular, their inclusion in
the analysis confirms the goodness of the neutrino oscillation hypothesis and
allows to exclude some alternative, exotic explanations such as neutrino decay,
flavour changing neutral currents, violations of the equivalence principle (at
least in their simplest forms), and also to discriminate, in principle, between
different neutrino oscillation models (nu_mu <-> nu_tau versus nu_mu <-> nu_s),
because of the difference in the matter effects.
