The World Meteorological Organization (WMO) Public Weather Services (PWS) Programme has the primary role of assisting National Meteorological and Hydrological Services (NMHSs) to develop or enhance their capacity to deliver services to the public, media, disaster managers and responders and other users in socio-economic sectors. The PWS Programme therefore assists NMHSs in their responsibility of providing warning and alerting services for the safety of life and livelihood, and property when threatened by extreme weather events such as heavy rain, heavy snow, strong winds, heat waves and extreme cold. In some countries, NMHSs are also responsible for issuing warnings for non-meteorological hazards such as tsunamis and volcanic ash. In order to disseminate the alerts to as many people as possible in a timely manner, NMHSs need to be able to send a consistent alert message through multiple communication channels such as radio, television (TV), land-line telephone connections, mobile phones, Internet, facsimile (fax) and sirens. The Common Alerting Protocol (CAP) standard is key to supporting such standards-based, all-hazards, all-media public alerting. The purpose of this publication is to provide step-by-step guidance for implementing the CAP standard in NMHSs.