  In this talk we will discuss the physics reach of the atmospheric neutrino
data collected by a future megaton-class neutrino detector. After a general
discussion of the potentialities of atmospheric neutrinos on general basis, we
will consider concrete experimental setups and show that synergic effects exist
between atmospheric and long-baseline neutrino data. Finally, we will show that
present Super-Kamiokande data already have the capability to allow for a direct
and unbiased measurement of the energy spectrum of the atmospheric neutrino
fluxes.
