This document defines surveillance and outlines its key elements and types. Surveillance involves the ongoing collection and analysis of health data to monitor diseases and health factors. It allows for the timely detection of disease outbreaks and evaluation of intervention strategies. The document discusses different definitions of surveillance from public health experts and organizations. It also covers the types of surveillance for different purposes, including outbreak detection and monitoring disease trends. Finally, it outlines steps for evaluating surveillance systems based on attributes like sensitivity, timeliness and whether appropriate actions are taken based on surveillance results.