Surveillance is the collection, collation, and analysis of data and the dissemination to those who need to know so that an action can result. This article describes the clinician's critical role in disease reporting and outlines the benefits of surveillance to clinical practice. Four commonly used systems of disease surveillance are notifiable disease reporting, laboratory-based surveillance, hospital-based surveillance, and population-based surveillance. We analyze the relative strengths and limitations of each of these systems and present current efforts to evaluate and improve surveillance activities.