  Cosmological simulations with gas dynamics suggest that the Lyman-alpha
forest is produced mainly by "small scale structure" --- filaments and sheets
that are the high redshift analog of today's galaxy superclusters. There is no
sharp distinction between Lyman-alpha clouds and "Gunn-Peterson" absorption
produced by the fluctuating IGM -- the Lyman-alpha forest {\it is} the
Gunn-Peterson effect. Lyman limit and damped Lyman-alpha absorption arises in
the radiatively cooled gas of forming galaxies. At $z~2-3$, most of the gas is
in the photoionized, diffuse medium associated with the Lyman-alpha forest, but
most of the {\it neutral} gas is in damped Lyman-alpha systems. We discuss
generic evolution of cosmic gas in a hierarchical scenario of structure
formation, with particular attention to the prospects for detecting 21cm
emission from high redshift HI. A scaling argument based on the present-day
cluster mass function suggests that objects with $M_{HI} >~ 5e11 h^{-1} \msun$
should be extremely rare at $z~3$, so detections with existing instruments will
be difficult. An instrument like the proposed Square Kilometer Array could
detect individual damped Lyman-alpha systems at high redshift, making it
possible to map structure in the high redshift universe in much the same way
that today's galaxy redshift surveys map the local large scale structure.
