Cyanobacterial toxins are produced by terrestrial- fresh-, brackish- and sea-water cyanobacteria of cosmopolitan occurrence. These toxins present acute and chronic hazards to human and animal health and are responsible for isolated, sporadic animal fatalities (mammals, fish, birds) each year. Human health problems are associated with the ingestion of, and contact with cyanobacterial blooms and their toxins. Modes of action of cyanobacterial neurotoxins, hepatotoxins and skin irritants are considered. Recent indications of the accumulation of cyanobacterial toxins in fish, their effect on crop plants and their association with the deaths of human dialysis patients are discussed. These findings and events indicate an incomplete understanding of the exposure routes of these natural toxins and the need for greater awareness of their occurrence and properties among users of waterbodies which are prone to cyanobacterial bloom development.