  It is difficult to describe in a few pages the numerous specific techniques
used to study absorption lines seen in QSO spectra and to review even rapidly
the field of research based on their observation and analysis. What follows is
therefore a pale introduction to the invaluable contribution of these studies
to our knowledge of the gaseous component of the Universe and its cosmological
evolution. A rich bibliography is given which, although not complete, will be
hopefully useful for further investigations. Emphasis will be laid on the
impact of this field on the question of the formation and evolution of
galaxies.
