The observation, in 1919 by A.S. Eddington and collaborators, of the
gra- tational de?ection of light by the Sun proved one of the many
predictions of Einstein's Theory of General Relativity: The Sun was the
?rst example of a gravitational lens. In 1936, Albert Einstein published
an article in which he suggested - ing stars as gravitational lenses. A
year later, Fritz Zwicky pointed out that galaxies would act as lenses
much more likely than stars, and also gave a list of possible
applications, as a means to determine the dark matter content of
galaxies and clusters of galaxies. It was only in 1979 that the ?rst
example of an extragalactic gravitational lens was provided by the
observation of the distant quasar QSO 0957]0561, by D. Walsh, R.F.
Carswell, and R.J. Weymann. A few years later, the ?rst lens showing
images in the form of arcs was detected. The theory, observations, and
applications of gravitational lensing cons- tute one of the most rapidly
growing branches of astrophysics. The gravi- tional de?ection of light
generated by mass concentrations along a light path
producesmagni?cation, multiplicity, anddistortionofimages, anddelaysp-
ton propagation from one line of sight relative to another. The huge
amount of scienti?c work produced over the last decade on gravitational
lensing has clearly revealed its already substantial and wide impact,
and its potential for future astrophysical applications.