The controversial question of whether the majority of the narrow absorption lines observed in QSO spectra represent cosmological intervening systems or ejecta from the QSO themselves is settled. QSO absorption line spectroscopy, initially a mere technique, has matured into an essential extragalactic research tool for understanding the content of the Universe at redshifts between 0 and 4, and beyond. The only previous important meeting devoted to "QSO Absorption Lines" was held in May 1987 at the Space Telescope Science Institute in Baltimore, Maryland, U.S.A. Since that time, nearly a decade ago, research has been ex­ tremely active in this now well-established field of astrophysics. Theoretical stud­ ies and simulations have taken advantage of the constant progress in computer technology, and during these last few years, the observational results have bene­ fited largely from the new facillities offered by the Hubble Space Telescope in the UV wavelength range and the Keck Telescope for high-resolution spectroscopy.