We have seen in the previous chapters that QSOs are valuable probes of the early Universe because they can be detected at large cosmological distances. The concomitant large lookback times provide a means of studying the Universe during the era when galaxy formation is expected to have occurred. QSOs can also be used in another way as a cosmological probe, namely as background sources against which we see intervening objects. Since the ‘sight lines’ to individual QSOs are of order Gpc, the chances of finding objects such as galaxies between us and any given QSO are non-negligible. Gas along the line of sight will produce absorption lines in the spectra of QSOs, and the redshift of these absorption lines zabs will reflect the cosmological distance of the absorbing cloud rather than that of the QSO (which will have emission-line redshift zem), so we can expect that QSO spectra will show absorption lines characterized by zabs < zem. An ‘absorption-line system’ consists of a number of absorption lines in a QSO spectrum that are all at very nearly the same redshift zabs and presumably arise in the same absorber. Thus, the objects probed are not the QSOs themselves, but the intervening material that produces the absorption spectrum. We include discussion of absorption-line characteristics (a) because some absorption lines actually appear to arise in material associated with the QSOs themselves, (b) because absorption by intervening material modifies the QSO spectrum we observe, and (c) because the study of QSO absorption lines has historically been closely associated with the study of the QSOs themselves.