  Analyses of QSO absorption lines are showing that HI content has evolved over
the redshift range z=5 to z= 0. The 21cm line measurements of the z=0 HI
content avoid several biases inherent in the absorption line technique, such as
the influence of evolving dust content in the absorbers, and will produce a
reliable measure to anchor theories of galaxy evolution. Examples of important
questions to be addressed by local HI surveys are: (1) is there a significant
population of gas-rich galaxies or intergalactic clouds that are missing from
the census of optically selected galaxies? (2) is there an adequate reservoir
of neutral gas to substantially prolong star formation at its present rate? and
(3) are there massive objects of such low HI column density that they can have
escaped detection in the ``unbiased'' HI surveys that have been conducted so
far?
