It will be seen that the QSO absorption lines have many potential uses in cosmology. We have real prospects that they can be used to study the gross properties of the interstellar gas in galaxies back to the time of their formation. The counts of absorption lines as a function of redshift will eventually provide an important cosmological test. Observations of correlated absorption, both in redshift and in nearby QSOs on the plane of the sky, are leading to important information on the evolution of the large-scale distribution of galaxies in space. Finally, the enigmatic Ly- clouds offer a unique opportunity to directly study the evolution of the intergalactic medium and the meta-galactic ionizing radiation flux and to provide information on primordial abundances of the elements.
Acknowledgements. I thank my co-workers, A. Boksenberg and C. Steidel, for allowing me to quote the results of our joint work in advance of publication. I also thank D. Tytler for providing me with details of his unpublished work. The work described in this paper was supported in part by the National Science Foundation under Grant AST84-16704.