© Oxford University Press, 2013. All Rights Reserved. Stuart L. Pimm and Clinton N. Jenkins explore why extinctions are the critical issue for conservation science. Extinctions are irreversible, unlike many other environmental threats that we can reverse. Current and recent rates of extinction are 100 times faster than the background rate, while future rates may be 1000 times faster. Species most likely to face extinction are rare; rare either because they have very small geographic ranges or have a low population density with a larger range. Small-ranged terrestrial vertebrate species tend to be concentrated in a few areas that often do not hold the greatest number of species. Similar patterns apply to plants and many marine groups. Extinctions occur most often when human impacts collide with the places having many rare species. While habitat loss is the leading cause of extinctions, global warming is expected to cause extinctions that are additive to those caused by habitat loss.