Abstract Several cyanobacteria species commonly forming mass occurrences (blooms) in fresh, brackish and marine waters produce toxins. They have caused death of animals all over the world and may be a health hazard for humans. The toxins are classified, according to the target of their toxic action, as hepatotoxins, neurotoxins and dermatotoxins. In fresh and brackish waters, the most frequently found hepatotoxins are cyclic hepta-and pentapeptides, microcystins and nodularins, respectively. In tropical waters, an alkaloid toxin cylindrospermopsin occurs. Three types of neurotoxins are known: anatoxin-a, anatoxin-a(S) and paralytic shellfish poisons (PSPs). In marine waters, neurotoxins and dermatotoxins have been reported. Most species producing these toxins are planktonic, but anatoxin-a is produced by benthic Oscillaroria and PSPs and dermatotoxins by Lyngbya. Microcystins are found worldwide; nodularins occur in brackish waters, mainly in the Baltic Sea, Australia and New Zealand. Both are structurall...