In consideration of the significant influence of thethreshold chloride concentration of reinforcing steel corrosion on the service lift of reinforced concrete structures, developments in research of threshold chloride concentration around the world are reviewed from the aspects of its expression, testing methods, and factors. Chloride threshold levels from previous research are presented, and the maximum allowable concentrations of chloride ions set as national standards in China are also provided. Some issues related to threshold chloride concentration are put forward for further research, including the influence mechanisms, testing methods, multi-factor coupling effect, and optimization of calculation models.