Statistics of QSO lensing and cosmological parameters

Statistics of gravitational lensing of QSOs by intervening galaxies provides a powerful tool to set constraints on cosmological parameters. In particular, lensing statistics are sensitive indicators of a nonvanishing cosmological constant. We review the present status of the subject and recent progress of theoretical works. Taking into account the current knowledge of early-type galaxies that effectively work as lenses, we present plausible lens models for calculating lensing statistics and give a detailed discussion on the possible value of a cosmological constant to reproduce the observed frequency of lensed QSOs and their image-separation distribution.