In the context of observational cosmology, the book begins with the basics of gravitational lensing, explaining how light is deflected as it passes the vicinity of mass. The focus however, is on applications related to gravitationally lensed quasars. A brief overview of the different classes of active galactic nuclei is given, as well as some still opened questions and unknowns about these objects.

Quasars are extremely bright active galactic nuclei and very important probes of the early Universe as they can be seen billions of light years away. Applications of gravitationally lensed quasars are described and show how they can be used to, e.g., detect dark matter, determine the Hubble constant H0, or act as a huge natural magnifying glass to probe very distant objects. Particular attention is given to a specific case of gravitational lensing, called microlensing. Microlensing can provide astonishing insights in the still mysterious inner parts of quasars with a spatial resolution about ten thousand times better than the capacities of the today 's best astronomical instruments, including the Very Large Telescope Interferometer.  less