Neutral hydrogen and dark matter in spiral galaxies

The first part presents a brief review of the main HI properties of isolate d, normal spiral galaxies and of the phenomena which seem to characterize a nd dominate their internal metabolism. In the second part attention is draw n to all those processes, such as tidal interactions, accretion and mergers , that depend on the galaxy environment and may play a significant role in galaxy formation and evolution. In the third part the observational evidenc e for the dark matter component of spiral galaxies is discussed.