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.