In this chapter, we outline the fundamental physiological processes that underpin the mechanisms of life in cyanobacteria. Specifically, we first explain the system that constitutes the light reaction in cyanobacterial photosynthesis, then move on to the dark reaction and carbon metabolic pathways, such as sugar metabolism and the tricarboxylic acid (TCA) cycle. We will also describe the reaction mechanism of nitrogen fixation found in many cyanobacterial strains and discuss nitrogen metabolism in cyanobacteria. Additionally, we briefly introduce phosphorus (P) metabolism and circadian rhythms in cyanobacteria. We believe that understanding these fundamental physiological processes will provide valuable insights into the salt stress response mechanisms in cyanobacteria, which will be addressed in later chapters.