Disease monitoring and surveillance systems (MOSSs) have become one of the major components of veterinary activity. Such systems are used to assess the existing levels of prevalence, the effectiveness of control programmes and, after disease eradication, to document the continued absence of disease from a given region or zone. With decreasing disease or infection prevalence, traditional approaches become less reliable and increasingly costly. The objective of this work was to summarize and discuss methodological issues related to veterinary (animal health) MOSSs. There are considerable inconsistencies in the use of the terms ‘monitoring’ and ‘surveillance’. Passive as well as active MOSS have their disadvantages when used for rare health–related events such as emerging and re–emerging diseases. There is a need for evaluation and improvement of these approaches. Integrated systems that call for the use of several parallel surveillance activities seem to be the favoured approach, and analytical methods to combine MOSS data from various sources into a population prevalence, or probability of disease freedom, are under development. The health and safety of the animal and human generations depends on our continuous ability to detect, monitor and control newly emerging or re–emerging livestock diseases and zoonoses rapidly. Uniform surveillance definitions, sound scientifically based approaches that use the resources and data available, and a pool of researchers and veterinary public health officials with sufficient training in epidemiology, are critically important to handle this challenging task.