Clinical liver transplantation became an established therapy of end-stage liver disease since the first at least medium-term successful liver transplantation in 1967. Clinical studies have played a major part in improving peri- and postoperative therapy in liver transplantation. In this article clinical studies of major impact are presented. Main topics are studies dealing with immunosuppressants, improvements in surgical techniques, viral infections and tumor diseases. Controlled randomized multicentric studies are rare; most of the studies are unicentric. Further studies in the fields of reducing side effects of immunosuppression, the introduction of monoclonal antibodies and improvement of the therapy of viral hepatitis would be helpful. These studies should be controlled, randomized and multicentric.