The two-component system is a signal transduction pathway in both of the prokaryotes and eukaryotes. Since there are only two components, histidine protein kinase(HPK) and response-regulation protein(RR), in the signal transduction pathway in prokaryotes, therefore, it is named the two-component system. However, further research, especially with the extensive identification of this system in eukaryotes, established that this system is composed of more than two components besides HPK and RR, showing diversity in its composition and signaling pathway. Usually, the two-component system is divided into several processes such as signal input, HPK auto-phosphorylation, RR phosphorylation, and signal output. The mechanism for the signal transduction of the two-component system and its role in the perception and transduction of osmotic stress are reviewed.