Surveillance of infections in blood donors and blood recipients can be useful for both transfusion medicine and public health. This thesis describes how an enhanced surveillance system for transfusion-transmissible infections has been established in England and Wales. Data from the surveillance system (1995 to 1999) have been used to monitor test performance and to describe the epidemiology of HBV, HCV and HIV in blood donors. The prevalence and incidence of HBV, HCV and HIV infections in blood donors have been monitored and were generally stable, and low compared to other countries and to other groups in the UK. HCV prevalence decreased throughout the 1990s. The exposure histories reported by infected donors indicate that donor selection largely succeeds in excluding high-risk groups, but also identify some failures in communication of, or compliance with, exclusion criteria. Diagnosed, reported, post-transfusion infections were rare and after investigation only 20% (21) were shown to have been transmitted by transfusion. The majority (52%) of reported transfusion-transmitted infections, and resulting deaths (3 of 4) were due to bacteria. The number of undiagnosed infections is not known but was estimated for HIV, HBV and HCV by calculations of the probability of infectious donations entering the blood supply due to true or false negatively to tests performed on donations prior to release. Various methods and assumptions have been used to investigate the robustness of these estimates and to develop an appropriate method for ongoing use in England and Wales. An enhanced surveillance system for transfusion-transmissible infections, that works in collaboration with national surveillance of infectious diseases and of non-infectious complications of transfusion, has been shown - despite some limitations - to provide data and analyses that have aided transfusion medicine and public health in England and Wales. This surveillance continues to develop and improve and further related work is planned.