The HI halos of spiral galaxies

The results of observational studies on the vertical HI density distribution and kinematics of the disks of spiral galaxies are reported and discussed. Attention is drawn to the presence of HI emission, unknown before and here referred to as the `beard’, which has anomalous structure and kinematics, different from the known cold HI disk. This component is extended and probably located in the halo region, it is rotating more slowly than the disk and shows radial inward motion. Its nature and origin -galactic fountain or infall of primordial gas?- are still a puzzle.