Gravitational lensing is a light-deflecting phenomenon when light rays pass through gravitational potential. It is so-called strong gravitational lensing when the lensing effects are strong enough to produce multiple images, arcs, or Einstein rings, primarily around high-density regions like centres of galaxies and galaxy clusters. Strong lensing by galaxies and galaxy clusters is a valuable tool for various applications in astrophysics and cosmology, e.g., investigating the nature of dark matter by measuring the mass distribution of lenses, constraining cosmological parameters via estimating the distances of lenses and sources, and as cosmic telescopes to magnify distant sources, etc.

Galaxy-scale strong lenses are mainly used to study mass structure and evolution fo galaxies, constrain the stellar initial mass function, and estimate cosmological parameters. Specifically, extended background sources can be lensed into multiple arcs or a complete Einstein ring; such systems are called galaxy-galaxy strong lenses and are applied to measure the total density profiles of the lenses as well as substructures. When the background sources are point-like, such as active galactic nuclei (AGNs), quasars, or supernovae, the lensing systems present multiple images, referred to as 'quads' or doubles when the observables are 4 or 2 detected point images, respectively. Considering the time domain information, strong lenses with multiple lensed point images are unique for constraining cosmological parameters.