Surveillance is the collection, analysis, and dissemination of results for the purpose of prevention. Surveillance tells us what our problems are, how big they are, where the solutions should be directed, how well (or poorly) our solutions have worked, and if, over time, there is improvement or deterioration. Surveillance is essential to successful sustained public health intervention for the purposes of prevention. Surveillance systems must be tailored to the specific disease or injury that is to be prevented. Surveillance should not be limited to the occurrence of death, disease, or disability. Public health is a multilevel cascade of activities involving recognition, evaluation, and intervention. Public health should include elements of experimentation as well as field implementation with evaluation. Surveillance is the mechanism to modify any element in the cascade based upon that element's contribution to prevention or lack thereof. Any element in the causal or intervention pathway is appropriate for surveillance as long as the monitoring of the element is useful in improving the prevention system. These elements include the occurrence of hazard and intervention as well as disease, death, or disability. Examples will be provided that demonstrate the roles of surveillance in the recognition of new diseases, the evaluation of the persistence of recognized problems, the estimation of the magnitude and trends of public health problems, and the provision of information to motivate intervention.