This 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 open questions and unknowns about these objects. Applications of gravitationally lensed quasars are described and particular attention is given to a specific case of gravitational lensing, called microlensing.
