Increasing global population and improvements in the standard of living mean that there is a rapidly increasing demand for animal protein with intensified animal production. The international movement of animals and animal products has been made cheaper and faster through improved transport infrastructure. Increasing human and livestock population has placed pressure on wildlife habitats, resulting in closer contact between wildlife, domestic animal populations and humans with spreading and re-emergence of diseases as consequences of these risk factors. Managing these 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. The approach can be much more cost-effective for some purposes, but if misused, it can lead to serious errors or it can be more expensive than traditional approaches.