This document provides an overview of public health surveillance. It defines surveillance as the ongoing collection, analysis, and interpretation of health data to inform public health programs and actions. The document outlines the historical origins of surveillance dating back to ancient Greece. It describes various types of surveillance including community-level surveillance, routine reporting systems, active and passive surveillance, sentinel surveillance, and surveys. It also discusses the integrated disease surveillance program in India and how it aims to strengthen surveillance systems at the state and district levels.