Cyanobacteria represent the earliest forms of prokaryotic life on Earth and played a critical role in shaping Earth’s early atmosphere. Prokaryotic in nature, cyanobacterial cells resemble other prokaryotes structurally but possess distinctive features, such as thylakoid membranes and photosynthetic light-capturing pigments, which enable them to perform oxygenic photosynthesis. In terms of their impacts on ecosystems, cyanobacteria are essential primary producers that form the base of the food web in many aquatic ecosystems and contribute to nutrient biological cycling, indicating they play an important role in ecological balance. In this chapter, we introduce the general background and characteristics of cyanobacterial cells to give an overview of these tiny prokaryotic powerhouses and the original pioneers of photosynthesis.