This chapter sets the stage for the first half of the book, the principles section. It outlines possible objectives of a system, followed by key steps in system development, data collection, data management, analyzing and interpreting data, disseminating and communicating information, and evaluating the surveillance system. Each step requires attention to a number of important details, which are introduced and addressed in detail throughout the first section of the book. Application of the principles is illustrated through an example using the National Notifiable Diseases Surveillance System in the United States.