Over 23,900 natural species are currently threatened with extinction, according to the International Union for Conservation of Nature's Red List of Threatened Species, including 41% of amphibians, 25% of mammals, and 13% of birds. This article collection, written by invited experts, looks at ecology and extinction risk-from the ancient DNA biomarkers that indicate environmental activity during historical extinctions, to modern day research in biodiversity loss. These Annual Reviews articles were freely available through December 2017 and are always available to subscribers.