Cyanobacteria, commonly known as blue-green algae, have played a fundamental role in shaping Earth’s evolutionary history, particularly through their contribution to the oxygenation of the atmosphere. This chapter explores their ancient lineage, morphological diversity, and central role in enabling complex life via oxygenic photosynthesis. From their emergence approximately 3.0 billion years ago to their involvement in the Great Oxidation Event, cyanobacteria have been instrumental in the development of life on Earth. Recent metagenomic studies have uncovered previously uncharacterized lineages, such as Melainabacteria and Sericytochromatia, revealing novel metabolic capacities and evolutionary relationships. This chapter also highlights cyanobacteria’s influence on global biogeochemical cycles and their adaptation to diverse ecological niches, including extreme environments. Molecular insights particularly from genomic analyses and horizontal gene transfer shed light on their evolutionary innovations and ecological versatility. Additionally, the environmental footprint of cyanobacteria, from harmful algal blooms to their potential role in astrobiology, is discussed. Overall, this chapter emphasizes the importance of continued research into cyanobacterial diversity and genomics to fully harness their biotechnological and industrial potential.