A definition of surveillance is as follows: “surveillance (ser-vâ1lens) noun. 1. Close observation of a person or group, especially one under suspicion. 2. The act of observing or the condition of being observed” ( The American Heritage Dictionary of the English Language , 3rd edition, Houghton Mifflin Company, 1992). The term “surveillance” is derived from the French word meaning “to watch over”. In public health, surveillance was originally developed as part of efforts to control infectious diseases, but the principles of surveillance can potentially be applied to other problems such as chronic diseases (for example, cancer and coronary heart disease), social problems (for example, drug addiction), and the threat of bioterrorism.1 Surveillance is a core activity in the practice of occupational health. Two broad groups of surveillance are commonly performed—hazard surveillance and health surveillance. While the focus of the former is hazards at the workplace, the latter type of surveillance pertains to the health of a person of group of workers. Both have important roles in occupational health practice and are complementary. The focus of this paper will be on chemical and biological exposures and related diseases. In many countries, occupational health concerns include psychosocial and ergonomic issues in the work environment and related problems and adverse health outcomes. These issues will not be addressed in detail in this paper, but surveillance programmes for such concerns have been developed, for instance, in Nordic countries. Hazard surveillance has been defined as “the process of assessing the distribution of, and the secular trends in, use and exposure levels of hazards responsible for disease and injury”.2 For this type of surveillance to be considered, a clear “exposure–health outcome” relation must already have been established. The surveillance of hazards should result in action to reduce exposure in workplaces where indicated. This …