Sixty-five million years ago dinosaurs ruled the warm Cretaceous Earth. Without warning, this world was swept away forever by the impact of an asteroid about 15 km in diameter, leaving a huge scar now called the Chicxulub crater in Yucatan, Mexico. This catastrophe set the stage for the ascendance of our own biological group, the mammals. Although the fact of this impact is now established beyond doubt, the precise means by which an impact could wipe out such a large fraction of the Earth\'s inhabitants is not fully understood. Recent study of the physical consequences of a large impact on the Earth have revealed a plethora of potentially disastrous effects, ranging from an immediate firestorm that ignited global wildfires to sulfuric acid aerosols, acid rain, and ozone depletion lasting decades. The extinctions caused by these physical traumas changed the way that the Earth\'s biosphere recycles carbon, leading to climatic changes lasting nearly a million years longer. Although no other major extinction in the past 500 million years can yet be tied unambiguously to an extraterrestrial impact, there is geological evidence of even larger impacts farther back in Earth\'s history, including the one that created the Sudbury ore body in Ontario more than a billion years ago. Concerns over the future possibility of such large impacts have led to a worldwide program to identify potentially threatening asteroids and has generated discussion of what humans might do to deflect such an asteroid if it is found.