Gravitational lensing is a well established field of astrophysics. It is well enough understood that it can be applied to other areas of astrophysics in order to tackle astrophysical problems under a new angle. Some applications of gravitational lensing concentrate on the study of the objects responsible for the deflection of light, the lenses. Others, aim at studying the stretched, distorted and (de)magnified images of the background objects, the sources. For example, stellar micro-lensing is used to probe the content of our own galaxy in dark low-mass stars, or micro-lenses. The weak distortions of very distant galaxies is used to detect indirectly and even to map what might be the largest lenses in the Universe: the Large Scale Structures. In many multiply imaged quasars, (micro)lenses are found within (macro)lenses: quasar micro- or milli-lensing provides us with information on the structure of both the sources and the lenses (see for example Schechter 2003; Wambsganss 2003).
Many of the applications of gravitational lensing, and in particular of quasar lensing, were known and described decades ago. They nevertheless only start now to be implemented on a systematic basis, taking advantage of the recent explosion of the number of large observatories that operate at high angular resolution and down to very faint magnitudes.
Quasar lensing helps us to study lenses and sources, but it also consists in a fantastic tool to study the space between the lenses and the sources ! While the light travels from the source to the observer, it is absorbed by the Inter Stellar Medium of the lens, and by the neutral gas of the Inter-Galactic Medium. The study of absorption lines in multiply imaged quasars provides us with information about the geometry of intergalactic clouds (Smette 2003). Last, but not least, multiply imaged quasars tell us about the size of the Universe, through the measurement of the so-called "time-delay" between the lensed images. This quantity is directly related to the mass distribution in the lensing galaxy and to the Hubble parameter H0. The measurement of H0 using lensed quasars is the topic of the present chapter.