Salmonella and Campylobacter are major human bacterial enteropathogens and the reported incidence is increasing in Great Britain despite efforts to control the problem. Most cases of disease are acquired by the ingestion of contaminated food and poultry are primary sources of infection. It is essential to reduce the carriage of these bacteria by poultry and the work contained within this thesis contributes to the understanding of the epidemiology of these infections in British poultry flocks. This knowledge is fundamental to the development of appropriate preventive measures. A national case control study of Salmonella enteritidis PT4 infection in poultry breeding flocks identified major risk factors for infection. Both the feed and the farm environment, including animal reservoirs, were shown to be sources of infection but there was no evidence of vertical transmission of infection from grandparent flocks. The most important factors that are likely to reduce the risk of infection in breeding flocks are thorough cleansing and disinfection of sites following a salmonella incident, strict hygiene measures between the site and the hatchery, effective isolation of the poultry unit from other domestic species and the use of heat treated poultry feed. Epidemiological studies of Campylobacter infection in broiler flocks revealed that Campylobacter jejuni infection was widespread within the British broiler industry. The national prevalence survey reported that at 5 weeks of age 45% of flocks were colonised (95% confidence limits: 36.9-53.1%) and a longitudinal study showed that this increased to more than 90% by 7 weeks of age. The environment, including contaminated drinking systems, appeared to be the main reservoir of Campylobacter infection for broiler flocks and the use of strict hygiene barriers at the entrances to broiler houses was shown to reduce the risk of infection. Interventions aimed at improving hygiene standards on sites are thus likely to reduce the prevalence of Campylobacter infection at slaughter.