West Nile fever is a zoonotic infection, caused by the West Nile virus (WNV), a virus of the Flaviviridae family. Its transmission takes place mostly through mosquito bites. It can lead to neurological disorders in humans and horses, with clinical symptoms of meningo-encephalitis (1-10% of cases). Numerous outbreaks observed recently have highlighted the importance of organizing a permanent and efficient surveillance system for WNV infection by appropriate information and intervention, in order to limit the number of neuro-invasive cases in humans and horses. Among surveillance tools, serological surveys (active surveillance) of sentinel animals (avian or equine) were carried out to strengthen basic passive surveillance (clinical surveillance). Active surveillance was implemented in avian species (call ducks and domestic poultry) from 2001 to 2007 in some of the departments on the French Mediterranean coastline. Horse sentinels were maintained in Guadelup since 2002. Starting from these two examples, we attempt to specify the sentinel function for WNV surveillance and to characterize the main benefits to be expected and the limitations of active surveillance.