Abstract: A quantum spin liquid (QSL) is a magnetically disordered many-body state at absolute zero temperature without any breaking of crystal symmetries, whose low energy physics exhibits features of fractionalization. Being beyond the Landau-Gintzburg paradigm of spontaneous symmetry breaking, it has been profoundly studied since the discovery of high-temperature superconductors, and has stimulated the development of topological order theory. Modern mathematical tools, such as the projective symmetry groups, are introduced to classify QSLs. The elementary excitations in a QSL are fractionalized spinons and emergent gauge photons or flux quanta, hence a gapped QSL has potential application in topological quantum computation. Theoretical approaches also include exactly solvable models, various numerical simulations, and quantum field theory. Experimentally, possible QSL candidates on triangular, kagomé, honeycomb lattices, and even three dimensional lattices, have been reported and investigated. To obtain QSLs, certain experimental conditions have been tried, such as high magnetic fields or high pressures. To detect their physical properties, various techniques have been applied, including nuclear magnetic resonance, neutron scattering, thermo transport, etc. The searching of definite QSL materials is still ongoing, and rapid progresses have been made recently. Researchers in China have made great efforts and are gradually playing a leading role in some specific directions. This research area is full of difficulties and challenges, as well as vitality and fascination.