  The relation between high redshift galaxies and QSO absorption systems is
discussed in the context of hierarchical galaxy formation. It is demonstrated
that imprints of the reionization history of the universe are detectable in the
$b$-parameter distribution of the \Lya forest at redshifts two to four,
favoring models in which hydrogen and helium are reionized simultaneously at or
before redshift five by a quasar-like spectrum of UV photons. Hydrodynamical
simulations including star formation, feedback due to supernovae and chemical
enrichment are also presented. Energy feedback in form of kinetic energy can
give rise to an efficient transport of metals out to distances of a few hundred
kpc. The observational signature of this metal transport mechanism compared to
simple homogeneous enrichment models is discussed. Finally, it is shown that
present day $L^*$ galaxies typically have several progenitors at $z\approx3$
spread over a few hundred kpc. These progenitors are closely associated with
damped \Lya and Ly-limit systems, and have velocity dispersions, luminosities
and colors comparable to U-dropout galaxies at $z\approx 3$.
