In 2015, 26% of the deaths of 5.9 million children who died before reaching their fifth birthday could have been prevented through addressing environmental risks – a shocking missed opportunity. The prenatal and early childhood period represents a window of particular vulnerability, where environmental hazards can lead to premature birth and other complications, and increase lifelong disease risk including for respiratory disorders, cardiovascular disease and cancers. The environment thus represents a major factor in children’s health, as well as a major opportunity for improvement, with effects seen in every region of the world.