Cyanobacteria or blue-green bacteria are a group of aquatic bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae, even though it is now known that they are not related to any of the other algal groups, which are all eukaryotes. Nonetheless, the description is still sometimes used to reflect their appearance and ecological role. Fossil traces of cyanobacteria have been found from around 3800 million years ago, making cyanobacteria some of the earliest living things known.

This group includes unicellular, colonial and filamentous forms. Some filaments have differentiated cells which are specialized for nitrogen fixation, called heterocysts. Each individual cell typically has a thick, gelatinous cell wall, which has a gram-negative stain. They lack flagella, but may move about by gliding along surfaces. Most are found in freshwater, but some are marine or occur in damp soil. A few are endosymbionts in lichens or of various protists, and provide energy for their host.

Photosynthesis in cyanobacteria generally uses water as an electron donor and produces oxygen as a by-product, though some may also use hydrogen sulfide as occurs among other photosynthetic bacteria. Carbon dioxide is reduced to form carbohydrates via the Calvin cycle. In most forms the photosynthetic machinery is embedded into folds of the cell membrane, called thylakoids. The large amounts of oxygen in the atmosphere are considered to have been first created by the activities of ancient cyanobacteria.

There are two main sorts of pigmentation. Most cyanobacteria contain chlorophyll a, together with various proteins called phycobilins, which give the cells a typical blue-green to grayish-brown colour. A few genera, however, lack phycobilins and have chlorophyll b as well as a, giving them a bright green colour. These were originally grouped together as the prochlorophytes or chloroxybacteria, but appear to have developed in several different lines of cyanobacteria.

Chloroplasts found in eukaryotes (algae and higher plants) most likely represent reduced endosymbiotic cyanobacteria. This endosymbiotic hypothesis is supported by various structural and genetic similarities. Primary chloroplasts are found among the green plants, where they contain chlorophyll b, and among the red algae and glaucophytes, where they contain phycobilins. Other algae likely took their chloroplasts from these forms by secondary endosymbiosis or ingestion. The question of whether chloroplasts had a single origin or developed in multiple lines has not yet been settled.