  The remarkable efficiency of the UVES spectrograph at the VLT has made it
possible to push high-resolution, high-S/N ground observations of the Ly-a
forest down to z~1.5, gaining new insight into the physical conditions of the
intergalactic medium and its evolution over more than 90% of the cosmic time.
The universal expansion, the UV ionizing background and the gravitational
condensation of structures are the driving factors shaping the number density
and the column density distribution of the absorbers. A (limited) contribution
of UV photons produced by galaxies is found to be important to reproduce the
observed evolutionary pattern at very high and low redshift. The Lyman forest
contains most of the baryons, at least at z>1.5, and acts as a reservoir for
galaxy formation. The typical Doppler parameter at a fixed column density is
measured to slightly increase with decreasing redshift, but the inferred
temperature at the mean density is increasing with redshift. The signatures of
HeII reionization and feedback from the formation of galactic structures have
possibly been detected in the Lyman forest.
