In the last three decades, disease surveillance has grown into a complete discipline, quite distinct from epidemiology. This expansion into a separate scientific area within public health has not been accompanied by parallel growth in the literature about its principles and methods. The development of the fundamental concepts of surveillance systems provides a basis on which to build a better understanding of the subject. In addition, the concepts have practical value as they can be used in designing new systems as well as understanding or evaluating currently operating systems. This article reviews the principles of surveillance, beginning with a historical survey of the roots and evolution of surveillance, and discusses the goals of public health surveillance. Methods for data collection, data analysis, interpretation, and dissemination are presented, together with proposed procedures for evaluating and improving a surveillance system. Finally, some points to be considered in establishing a new surveillance system are presented.