Hepatocellular carcinoma (HCC) is the most frequently occurring liver carcinoma world-wide. Clinical and molecular medical analyses have produced a considerable amount of information about liver carcinogenesis. Loss of heterozygosity (LOH) analyses have revealed several chromosomal loci harboring potential tumor suppressors. These data support the idea that deletion or inactivation of tumor suppressors including RB, p53, BRCA2, E-cadherin and other candidate genes seem to be common events in HCC development. Factors associated with cell cycle regulation via the Wnt- and MAPK/ERK signaling pathways are frequently deregulated in hepatocarcinogenesis. Aberrant activation of telomerase also occurs in precancerous as well as cancerous lesions in HCC patients. To characterize the wide variety of genetic events that occur in HCC, mRNA expression has been compared in HCC and non-cancerous liver tissues, and several differentially expressed genes have been identified. Hepatitis B and C viruses are the main risk factors for HCC, and indeed some accessory functions of viral products seem to contribute to tumor development; however, whether they have a direct carcinogenic effect has not yet been established.