Quasars are the most energetic and distant members of active galactic nuclei. Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars than to extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way. The nearest known quasar, Markarian 231, is about 581 million light-years from Earth, while others have been discovered as far away as UHZ1, roughly 13.2 billion light-years distant. Quasars are noteworthy for providing the first demonstration of the phenomenon that gravity can act as a lens for light.