Even though Einstein remained sceptical with regard to the observability of gravitational lenses, they are not only a nice sideline of general relativity, but constitute an undispensable tool for todays astrophysics.

In the face of hundreds of observed gravitational lensing effects they serve these days for e.g. determining the mass distribution of galaxies and galaxy clusters, the dynamics of the expanding universe, and for the detection of extra-solar planets.