Managing disease threats poses enormous challenges and requires good quality information: what diseases exist; where they are found; what impact they are having; which populations are at risk; how we can prevent, control or eradicate these diseases. Animal disease surveillance plays a central role in providing this information. Risk-based surveillance is not a particular technique; rather, it describes a general approach to undertaking disease surveillance. The principle is simple and self-evident: the most efficient way to find disease is to survey the animal populations that are most likely to be affected. This is in contrast to the more traditional statistically-based approach of taking representative samples from a population. While the idea of risk-based surveillance is simple, the implications are complex.