Despite intense interest in Clostridium difficile-associated diarrhoea (CDAD) over the last 15 years, the epidemiology of CDAD is still poorly understood. Initially it was thought that the pathogenesis of CDAD involved simple overgrowth of the gut by resident C. difficile following exposure to antibiotics. While this may be so in a small percentage of cases, the majority of infections are acquired from exogenous sources. C. difficile has been isolated from the gastrointestinal tract of several animals, including domestic pets, and from soil and sediment samples. At present the majority of cases of CDAD represent hospital-acquired infections and it is not unreasonable to assume that the hospital environment is a major source of C. difficile. Investigations into community-acquired CDAD suggest that this problem may be more common than at first thought and may represent a reservoir of infection in the community. The exact role of reservoirs of infection other than the hospital environment remains to be determined.