Gravitational lenses as astrophysical tools

Observations of GL systems can provide a wealth of astrophysical information. As F. Zwicky noted as early as one-half century ago [ZW37.1], GLs (1.) furnish an additional test of General Relativity; (2.) allow us to see fainter or more distant sources; and (3.) provide a means to estimate the mass of galaxies, of clusters of galaxies, and of unseen, massive aggregates of dark matter.