Abstract

Strong gravitational lensing has transformed into a powerful astrophysical probe in the past thirty years. Strong lensing occurs when an astrophysical source is magnified and distorted into multiple images by a foreground massive object through a chance alignment along the line of sight. Such phenomena span a wide range of mass scales, from small galactic substructure of 107 solar masses up to galaxy clusters of 1015 solar masses, and thus yield a great number of astrophysical and cosmological applications including galaxy formation and evolution, dark matter substructures and halos, lenses as natural telescopes, galaxy cluster physics, high-redshift sources, reionization and dark energy. The number of lenses has increased to hundreds in the past decade, and current and future surveys are guaranteed to provide tens of thousands more. The wealth of strong lensing data will provide a grand and unique opportunity to understand our Universe. Analyses will no longer be statistically limited. A concerted effort between observers and theorists will be needed to control systematics and reap the rewards from the large lens sample size.