Heat or hot weather that lasts for several days, often referred to as “a heatwave” can have a significant impact on society, including a rise in mortality and morbidity. Heatwaves also place an increased strain on infrastructure (power, water and transport). Clothes and food retailing, tourism and ecosystem services can also be affected, such that there may be socioeconomic “winners and losers” from heatwave events. In some instances, heatwaves may even trigger social disturbances at a number of levels.The impacts of heatwaves can be great and sometimes catastrophic, as manifested by the large number of heat-related deaths recorded across Europe in July and August 2003, and the Russian Federation in July and August 2010. While the effects of heat may be exacerbated in cities, due to the urban heat island (UHI) effect, the livelihoods and social well- being of non-urban communities can also be severely disrupted during and after periods of unusually hot weather.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).