Author name: W. Berry Lyons

In a very short period of time, the majority of the human population has become urban, and by 2050 two out of every three people in the world will live in cities. Urban areas are extremely important socially, economically, and culturally, but they also have a profound impact on the environment. In that context, this issue of Elements considers the geochemical significance of 21st-century cities and some of the unprecedented challenges they face.