The authors describe a comprehensive system for public health surveillance for the United States based on a network of data systems ranging from population surveys and physician-based records to electronically linked laboratory and administrative data. They also discuss traditional uses of surveillance data, legal and ethical issues associated with using data from any surveillance system (particularly the tension between individual privacy and the public right to a healthful environment), and factors impeding the development of a comprehensive system. Just as provisional data on notifiable diseases are critical in protecting communities from disease, data from other information systems should be applied to prevention practice with the same urgency. The major barriers to a successful comprehensive, nationwide, integrated public health surveillance and information system are a lack of appreciation for the value of high-quality provisional surveillance data and a weak societal commitment to public health.