Recent heat waves around the world are prompting the World Meteorological Organization (WMO) to step up its efforts to strengthen early heat warnings and integrated heat wave action plans. Heat poses a rapidly growing health risk due to climate change, increasing urbanization and demographic shifts in countries with ageing populations. Simultaneous heat waves with maximum temperatures above 40°C and high night temperatures for several days, as seen in July in the Mediterranean, North America, parts of Asia and across North Africa, have increased sixfold since the 1980s. As heat waves are among the deadliest natural hazards, hundreds of thousands of people die each year from preventable heat-related causes. The impacts on human well-being, the economy and the natural and built environment are severe. Tried, tested and repeatedly updated response strategies and communication plans are needed to target the general population and vulnerable groups such as older adults, out-of-work workers or socially disadvantaged populations.