  WSRT observations have provided a first inventory of the incidence of HI 21
cm line absorption associated with AGN at redshifts up to z=1.0. There is a
large range in line depths, from tau=0.44 to tau less than 0.001, and a
substantial variety of line profiles, from Gaussians of less than ten km/s to
more typically a few hundred km/s, as well as irregular and multi-peaked
absorption, sometimes spanning many hundreds of km/s. The chance to detect
appreciable HI absorption is greatest in the most compact radio sources, GPSs
and CSOs, where it can occur in circumnuclear ``disks'' or ``tori'', as well as
in gas enveloping jets and hot spots; inferred densities range at least between
10 per cubic cm and 10000 per cubic cm. But HI absorption occurs also in some
CSSs, perhaps associated with jet-cloud interaction regions, and in quasars
with a large optical reddening. VLBI observations at the unusual UHF
frequencies of redshifted HI 21 cm can give unique ``sight''lines into the
physics and evolution of young radio sources and their inner galactic medium.
