Towns and cities generally exhibit higher temperatures than rural areas for a number of reasons, including the effect that urban materials have on the natural balance of incoming and outgoing energy at the surface level, the shape and geometry of buildings, and the impact of anthropogenic heating. This localized heating means that towns and cities are often described as urban heat islands (UHIs). Urbanized areas modify local temperatures, but also other meteorological variables such as wind speed and direction and rainfall patterns. The magnitude of the UHI for a given town or city tends to scale with the size of population, although smaller towns of just thousands of inhabitants can have an appreciable UHI effect. The UHI “intensity” (the difference in temperature between a city center and a rural reference point outside the city) is on the order of a few degrees Celsius on average, but can peak at as much as 10°C in larger cities, given the right conditions. UHIs tend to be enhanced during heatwaves, when there is lots of sunshine and a lack of wind to provide ventilation and disperse the warm air. The UHI is most pronounced at night, when rural areas tend to be cooler than cities and urban materials radiate the energy they have stored during the day into the local atmosphere. As well as affecting local weather patterns and interacting with local air pollution, the UHI can directly affect health through heat exposure, which can exacerbate minor illnesses, affect occupational performance, or increase the risk of hospitalization and even death. Urban populations can face serious risks to health during heatwaves whereby the heat associated with the UHI contributes additional warming. Heat-related health risks are likely to increase in future against a background of climate change and increasing urbanization throughout much of the world. However, there are ways to reduce urban temperatures and avoid some of the health impacts of the UHI through behavioral changes, modification of buildings, or by urban scale interventions. It is important to understand the physical properties of the UHI and its impact on health to evaluate the potential for interventions to reduce heat-related impacts.