Cyanobacterial blooms occur worldwide and present an increasing problem due to eutrophication of lakes. Microcystins, especially microcystin-LR, are microcyclic heptapeptide hepatotoxins and are the most common and potent toxins associated with cyanobacteria. Microcystin is rapidly taken up by hepatocytes through carrier-mediated transport. Once in the hepatocyte, microcystin causes structural damage to the cell indirectly by inhibiting protein phosphorylases 1 and 2A, which are needed for regulation of structural proteins of the cell. Acute liver hemorrhage and death occur with high doses of microcystin-LR, which is also a potent tumor promoter in laboratory rats. The significance of microcystin to human health has been debated; however, poisoning in humans has occurred due to contaminated dialysis water. Microcystin in contaminated drinking water may be the cause of elevated rates of primary liver cancer in some areas of China. Problems with hepatotoxic cyanobacteria have been most seen in livestock. Treatment of confirmed microcystin toxicosis in livestock is likely to be unrewarding, so prevention is important. Wild mammals, birds, fish, insects, and microinvertebrates may also be affected by microcystin.