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 structurally related and act identically as inhibitors of protein phosphatases 1 and 2A and potential tumor promoters. Microcystins have been characterized from species of Microcyslis, Oscillaloria, Anabaena and Nosloc and nodularins from Nodularia spull1igena Mertens. Cylindrospermopsin produced by Cylindrospennopsis raciborskii (Woloszynska) Seenaya et Subba Raju and Umezakia natans Watanabe is a cytotoxin that primarily affects the liver and kidney in mammals. All neurotoxins block the neurotransmission: anatoxin-a by mimicking the effect of acetylcholine, anatoxin-a(S) as an anticholinesterase and PSPs by blocking the sodium channels. Anatoxin-a is produced by Anabaena, Oscillaloria and Aphanizomenon, anatoxin-a(S) only by Anabaena and PSPs by Aphanizomenon (USA, Portugal), Anabaena circinalis Rabenhorst ex Bornet et Flahaut (Australia) and Lyngbya wollei comb. nov. (USA). Structural variation, especially in microcystins and PSPs, is common. Presently about 50 different microcystins are known and about 10 PSPs have been shown to be produced by cyanobacteria. Environmental factors have effects on toxin production. In general, microcystins, nodularins and anatoxin-a are produced at maximum amounts when the conditions are favourable for growth. Studies on the biogenesis of toxins, combined with the increasing interest in using molecular methods to characterize toxic and non-toxic strains will expand our understanding of toxin production in the near future.