Cyanobacteria are ubiquitous photoautotrophic prokaryotes commonly found in aquatic and terrestrial ecosystems. Some can even thrive under immense oxidative stress in extreme environments, such as hot/cold deserts and hot springs. Formation of reactive oxygen species (ROS) in cyanobacteria not only occurs in the respiratory metabolism, but also involved in the receptor–side of the electron transport chain during photosynthesis. This book discusses the ecological importance of cyanobacteria. It also discusses some biotechnological uses and risk management.