Since I want to describe some new results, I shall not dwell on the history of the subject as is customary on these occasions. Instead, I shall confine myself to a brief summary of the present state of affairs, without saying how we got there. Thus, I shall largely resist the temptation to dispense remarks of a vaguely philosophical character of the kind which M. J. Rees once described to me as "unspecific wisdom". Nevertheless, it is clear from the impressive number and variety of the talks and posters at this Workshop that the field of QSO absorption lines is just beginning a new phase, one in which the lines are actually being used to obtain information on the distant Universe. Accordingly, we shall hear reports on the properties and distribution of distant galaxies, on the intergalactic medium and the meta-galactic radiation field, all obtainable in no other way.
The QSO absorption lines are currently divided into three distinct categories:
The heavy-element redshifts include two important sub-categories: the damped Ly- systems, which Wolfe (1988) will discuss, and the Lyman limit systems.
It is almost universally agreed that the Broad Absorption Line redshifts, whose properties will be described by Turnshek (1988), are caused by expanding gas which has been ejected from the QSO in question. I shall not say anything further about these systems. On the other hand, the origin of the first two categories has been more controversial. There are several reasons for believing that the majority of the sharp absorption lines are formed in cosmologically distributed, intervening objects. The strongest pieces of evidence are as follows.
An exception to this generalization is provided by certain heavy-element absorption complexes which are found close to the emission redshift in certain categories of radio-QSOs (Foltz et al. 1986; Barthel 1987; Foltz et al. 1988). These complexes, which may be related to the broad absorption troughs, almost certainly arise in gas which has been ejected by the galaxy containing the radio source.