This paper reviewed on the studies on the effects oh microcystins on human health. So far, there have been several important historical events related with this. First, scientists revealed for the first time that microcystins could potently inhibit protein phosphatase 1 and 2A in 1990, which was the most important molecular basis for the toxicity of microcystins. Second, human intoxications by MCs (-LR, -YR and -AR) caused deaths of 52 patients at Caruaru dialysis centers in Brazil in 1996. Third, high incidence of primary liver cancer in southeast of China was related with MC contamination of drinking waters. Fourth, World Health Organization provided a provisional guideline value of I mu g/L for drinking water in 1998. Recently, microcystins were identified for the first time in the serum of a chronically exposed human population (fishermen at Lake Chaohu, South China) together with indication of hepatocellular damage. Conclusively, there have been substantial evidences to indicate the harmful influences of microcystins on human health, and therefore the toxic effects of microcystins on human being can not be ignored especially in nowadays with ever-increasing eutrophication and frequent occurrence of toxic cyanobacterial blooms.