Neutral Hydrogen in Spiral Galaxies

I will present a brief review of the main HI properties of isolated, normal spiral galaxies and of the phenomena which characterize and dominate their internal metabolism. In particular, I will present and discuss new results on the HI halos and on the HI disk-halo connection. Finally I will draw attention to all those processes, such as tidal interactions, HI accretion and mergers, which depend on the galaxy environment and may play a significant role in galaxy formation and evolution.