The incidence of liver cancer varies widely throughout the world, with high rates in sub – Saharan Africa, eastern and southeastern Asia, and Melanesia and a low incidence in Northern and Western Europe and the Americas. Primary cancers of the liver in adults are of two main histological types: hepatocellular carcinoma, which is derived from hepatocytes, and cholangiocarcinoma, which is derived from the epithelial lining of the intrahepatic bile ducts. Hepatocellular cancer is a frequently occurring tumor in individuals in many developing countries, where several important risk factors have been demonstrated, including chronic infection with hepatitis B and C viruses and other environmental factors, such as exposure to aflatoxin, consumption of alcohol, and cigarette smoking. By contrast, cholangiocarcinoma is less common, accounting for only 7.7% of malignant tumors of the liver in the United States. However, in parts of Southeast Asia, cholangiocarcinoma occurs more frequently; it is responsible for more than 60% of liver tumors in northeastern Thailand. The geographic distribution worldwide coincides with endemic areas of the liver flukes, Opisthorchis viverrini and Clonorchis sinensis. The interaction between genes and the environment and the interplay of environmental factors, which include diet and other lifestyle parameters, illustrate the complexity underlying susceptibility.