The role of surveillance in preventing occupational diseases and injuries was discussed. The discussion considered the nature of surveillance and its relationship to the hierarchy of prevention as practiced in public health. Surveillance as it is used in occupational health can be defined as the systematic collection and analysis of information concerning hazards, diseases, or injuries for the purpose of preventing occupational diseases or injuries. Surveillance programs can be either case based or rate based. In case based surveillance, the occurrence of a case, such as a Sentinel Health Event, indicates a failure of prevention. Investigation of cases results in the cause for the failure being identified and mitigated. Rate based surveillance is based on determining the rate of occurrence of a disease or injury outcome and comparing it with the expected rate. Disease or injury excesses over that expected are regarded as a cause for investigating the failure of prevention. The traditional hierarchy of prevention in public health would be expanded to include other techniques for preventing occupational disease and injury, such as environmental monitoring for a hazard, biological monitoring for evidence of absorption of a toxicant, medical screening for preclinical evidence of disease, medical diagnosis when the disease becomes symptomatic, medical treatment, and rehabilitation. Surveillance is not an intervention, but rather represents a technique for collecting, analyzing, and using information about the intervention technique. If surveillance is added to the continuum of prevention, the continuum becomes dynamic in that collection of information at any one level in the continuum can be used to modify the effect of any of the adjacent levels.