This Guidance has been developed jointly by WMO and WHO to outline for practitioners in both NMHSs and National Health Services (NHSs) the issues surrounding the general heat–health problem and present how an understanding of the biometeorology, epidemiology, public-health and risk-communication aspects of heat as a hazard can be used to inform the development of an HHWS as part of a wider HHAP. The Guidance places emphasis on the practical aspects of HHWSs at a generic level and is not intended to be prescriptive. The Guidance has been produced to have global applicability. It has drawn on expert opinion and the acquired experience of a wide range of people and institutions involved in the development of warning systems and heat plans. In particular, it has been enriched by information contained in the US Environment Protection Agency’s Excessive Heat Events Guidebook and reports of the projects funded by the European Commission under its fifth (1998–2002) and sixth (2002–2006) framework programmes: Assessment and Prevention of Acute Health Effects and Weather Conditions in Europe (PHEWE), Climate Change and Adaptation Strategies for Human Health (cCASHh) and Improving Public Health Responses to Extreme Weather/Heat-Waves (EuroHeat).