Tropical marine ecosystems are characterized by a specific cyanobacterial flora, temperature most probably being the major factor limiting the geographic distribution of the species. When compared with the open ocean, the highest biodiversity of cyanobacteria is observed in the littoral zones where they form intertidal and infralittoral mats, live as endoliths in carbonate substrates or form symbiotic associations, especially with sponges and ascidians. Their diversity, which especially in the less accessible infralittoral is still largely unknown, is also a source of diverse bioactive compounds, some of which are important as herbivore deterrents. As photosynthetic organisms, cyanobacteria sensu lato (including Prochlorophyta) are important contributors to benthic and open ocean primary production, but their main role in the tropical marine ecosystems appears to be as nitrogen fixers. Of primary importance in the often oligotrophic tropical oceans is the non-heterocystous, planktonic bloom-forming Trichodesmium, which probably represents a major nitrogen source for the marine and global nitrogen cycle.