Gravitational Lensing

This is an exhaustive review of our theoretical and observational knowledge of gravitational lensing 10 years after the discovery of the first lensed quasar, Q0957+561. Gravitational optics, optical, infrared, and radio observations of quasar-lens candidates, microlensing, arcs in clusters of galaxies, and radio rings are presented. In particular, the continuing survey of quasar-lens candidates, the new measurement of the time delay in 0957+561, the suspended microlensing effect through the galaxy 2237+030, as well as the discovery of new arcs and the measurement of new redshifts for two of them are presented. Numerous papers on the modelling of arcs and rings show how it should be possible to probe dark matter with these unexpected gravitational telescopes. Finally, tables summarize all the lens candidates we know today.