  Neutral hydrogen traces gravitational potentials. In the nearby universe,
21cm emission-line surveys show that the bulk of the HI resides in well-formed,
optically-luminous galaxies. At high redshift, 21cm line absorption against
background radio quasars occurs in gas-rich systems identified with the highest
HI column densities -- the ``damped Lyman alpha'' quasar absorption-line
systems. High spatial-resolution observations of the redshifted 21cm line
absorbers measure sizes and kinematics of the neutral absorbers.
