This guide was prepared as a collaborative effort between the COMET® Program of the University Corporation for Atmospheric Research (UCAR), and staff of the National Oceanic and Atmospheric Administration (NOAA) as a U.S. Contribution to the World Meteorological Organization (WMO) Voluntary Cooperation Program (VCP). Principal authors were Christopher D. Hill (COMET) and Firoz Verjee (NOAA), with significant input from Curt Barrett (NOAA-retired). This guide is intended to help national hydro-meteorological services in developing a concept of operations for a flash flood early warning system. It includes chapters on Flash Flood Science, Flash Flood Forecasting Methods, Monitoring Networks, Technology Infrastructure, Warning Dissemination and Notification, and Community-based Disaster Management, and several examples of Flash Flood Early Warning Systems. This Guide will encourage and assist countries and communities to develop early warning systems needed to save lives and protect property from the sudden threat of flash floods.