The aim of an early detection system is to ensure detection of the introduction of, or sudden increase in the incidence of, any disease of aquatic animals that has the potential of developing to epizootic proportions and/or causing serious socio-economic consequences. It embraces all initiatives that lead to improved awareness and knowledge of the distribution and behavior of disease outbreaks (and of infection) and that allow forecasting of the source and evolution of the disease outbreaks and the monitoring of the effectiveness of disease control campaigns. The key components of a national early detection system are given in Box 2.