Abstract
No evidence has been found for
distributed
H I at red shifts
z
< 2, although clumped H I is found in the absorption spectra of quasars. Observations of nearby groups of galaxies indicate the presence of substantial amounts of H I lying outside the optical confines of the galaxies. Some of this material is in the form of tidal bridges and tails and some is condensed into discrete intergalactic clouds. H I observations have also been used to investigate with higher precision than hitherto the virial mass of groups of galaxies. Two such groups have a lower mass and a lower mass light ratio than obtained from equivalent optical studies. The origin of this difference is not fully understood, but it may have consequences for the conventional methods of determining the mass density of the Universe. Finally, searches for H I in the epoch of galaxy form- ation (
z
= 3-10) are described. Limits have been set on the masses and the volume density of protoclusters at
z
= 3.3 and 4.9. This appears to be an important area for further observational and theoretical study.