In this chapter, we endeavor to present a comprehensive overview of research and developments related to the formation, micro/nanostructure, and properties of C–S–H. By integrating insights from different scales, from atomistic scale to microscale, we aim to provide an elementary understanding of C–S–H in this chapter. Some basic concepts related to C–S–H micro/nanostructure will be introduced here. In particular, the classical chemical-structural and microstructure models of C–S–H are summarized. This makes it easier to discuss the cryogenic stability of C–S–H in following chapters.