This document describes the evaluation of epidemiologic surveillance systems. Its purpose is to promote the best use of public health resources through the development of effective and efficient surveillance systems. It can serve as a guide for persons conducting their first evaluation and as a reference for those who are already familiar with the evaluation process. Epidemiologic surveillance is the ongoing and systematic collection, analysis, and interpretation of health data in the process of describing and monitoring a health event. This information is used for planning, implementing, and evaluating public health interventions and programs. Surveillance data are used both to determine the need for public health action and to assess the effectiveness of programs.