  I review the capabilities of H$\alpha$ observations to constrain some aspects
of the current models of the interstellar medium. In particular, it is shown
that turbulence is a necessary ingredient of any viable model, since most of
the energy produced by supernova explosions and ionizing radiation is stored in
kinetic form in the ISM. Various forms of turbulent energy dissipation,
including cloud collisions, are analyzed. Two additional aspects, concerning
the existence of galactic fountains and their relation with High Velocity
Clouds, and the extended ionized layer of spiral galaxies are discussed; some
crucial experiments are suggested.
