Surveillance of healthcare-associated infections is central to healthcare epidemiology and infection control programmes and a critical factor in the prevention of these infections. By definition, the term ‘infection prevention’ implies that healthcare-associated infections may be preventable. The purpose of surveillance is to provide quality data that can be used in an effective monitoring and alert system and to reduce the incidence of preventable healthcare-associated infections. This article examines the purpose of surveillance, explains key epidemiological terms, provides an overview of approaches to surveillance and discusses the importance of validation.