The guide is presented in two parts:<br> <b>Part 1. Principles of Operational Monitoring:</b> Describes the key principles of operational monitoring, alongside the types of operational monitoring that may be performed and the information required within an OMP.<br> <b>Part 2. Operational Monitoring Plan Development:</b> Describes the stepwise development of an OMP for a water supply system, including the source, water treatment, intermediate storage, distribution and household. For illustration purposes, practical guidance is provided using a specimen water supply system considered to be representative of a conventional small- to medium-sized supply in a lower resource setting. This template may be used to develop system-specific OMPs for individual water supply systems.