Soon after the isolation of the hepatitis C virus (HCV) genome in 1988 it became evident that HCV is the most important cause of non-A, non-B-hepatitis. In recent years the structure of this (+)-stranded RNA-virus, the different genotypes of HCV and the replication in hepatic and extrahepatic sites have been investigated. HCV has a remarkable degree of genetic heterogeneity, mutates rapidly, leading to the simultaneous coexistence of different genoms in the same individual (quasispecies) and most likely to the generation of neutralization escape mutants. The cytotoxicity of the hepatitis C virus appears to be mainly immune-mediated. This review article summarizes basic, diagnostic and clinical aspects of acute and chronic HCV-infection, association with other diseases and complications such as liver cirrhosis and hepatocellular carcinoma. Interferon-alpha has been shown useful in normalizing serum aminotransferases and decreasing liver inflammatory lesions in about half of the patients with chronic hepatitis C. However, relapses after the cessation of interferon-alpha are frequent, leading to a sustained response in less than 30% of treated patients. Several clinical and biochemical parameters for response to interferon-alpha have been proposed. In patients with orthotopic liver transplantation due to progressive chronic hepatitis C and decompensated liver cirrhosis, reinfection of the donor organ frequently occurs. However, in transplanted patients under immunosuppression the course of hepatitis C reinfection is usually mild. Due to screening programs of blood and blood products the incidence of posttransfusion-acquired hepatitis C has declined. However, further efforts in understanding the transmission of community-acquired hepatitis C are necessary. The development of a hepatitis C vaccine will be difficult due to the high degree of viral genetic heterogeneity.