Aflatoxins, especially aflatoxin B₁, are potent hepatocarcinogens that induce liver tumors in many species of animals, including rodents, nonhuman primates, and fish. Human primary liver cancer, mainly hepatocellular carcinoma, is one of the most common diseases in Asia, Africa, and in populations of Asian- and Hispanic-Americans. Over the past 40 years there have been extensive efforts to investigate the association between aflatoxin exposure and human liver cancer. These studies have been hindered in earlier years by the lack of adequate biomarkers and dosimetry data on aflatoxin intake, excretion, and metabolism in people, as well as by the general poor quality of world-wide cancer morbidity and mortality statistics. Many studies carried out in the past decade have incorporated the molecular analysis of the cancer gene targets and aflatoxin-specific biomarkers, which have spurred efforts to assess aflatoxin exposure and human liver cancer risks. These molecular epidemiological studies eventually led to the reclassification of naturally occurring aflatoxins to a Group I human carcinogen by the International Agency for Research on Cancer in 1993, and the evaluation was reaffirmed in 2002. Current research in the field mainly focuses on studying interactions between aflatoxins and viral infections (hepatitis B/C viruses) and preventions of both aflatoxin exposure and viral infections.