At the beginning of the 21st century, GOL’TSMAN et al. started the research field of superconducting nanowire single photon detector (SNSPD). After researches and developments in the past twenty years, SNSPDs have become a type of single-photon detectors with unprecedented comprehensive performances, and have been applied in quantum and classical faint-light detection. In this paper, the important researches and developments of SNSPDs in the past twenty years were reviewed from the following nine aspects: Performance metrics, device physics, superconducting films, device structures, fabrication, optical coupling, electronic readouts, cryogenic systems, and applications. Then, the future directions for research and development were discussed.