Climate hazards arise through interactions of weather-related shocks, vulnerability, and exposure. The atmosphere is warming and population growth is increasing, setting the stage for potentially explosive increases in impacts. Of all weather hazards, heat waves tend to be the most immediate, and often the most deadly. Unfortunately, relatively small changes in air temperatures can lead to large increases in the frequency of extreme heat waves. This chapter uses 1880–2019 monthly and 1983–2016 daily temperature estimates to explore observed increase in extreme temperatures. Exceptional warmth, over more than 20 perent of the Earth's surface, has become the new norm. Warmer-than-ever conditions prevailed in 2015 through 2019. Over this same time period 71 extreme-temperature disasters affected 4.5 million people, resulting in 9,916 deaths, 90,014 injuries, and $1.8 billion losses. These exceptional temperatures threaten the Earth's basic ecosystem services: fisheries, coral reefs, and CO2-absorbing rainforests. Analysis based on a new very high-resolution data set identifies very large increases in the number of people exposed to very warm heat waves. Between 2000 and 2016, the number of heat wave exposure events has increased by approximately 15 billion people-days. Climate change projections for 2050 indicate further increases of ~70 billion. A sidebar describes a climate attribution study on Hyderabad, India, in 2015.