Drawing together leading observational astronomers, atmospheric researchers and public policy analysts, including Edward Teller, David Morrison, and Eugene Shoemaker, the discussions in this volume provide a detailed look at the probability and implications of an impact of an asteroid or comet with the Earth. The chapters are based on discussions held at the Erice International Seminar on Planetary Emergencies in 1993, which reached the following conclusions: cosmic impacts have already played a significant role in the evolution of the Earth itself and of life on the Earth; while the threat of such a large impact is minuscule in any one year, the consequences are so grave that one should continue to study the threat; a small impact is considerably more probable and carries the additional threat of being mistaken for a nuclear explosion at a time of international tension, with dire consequences; and that it is necessary to gather data on objects near the Earth, or in orbits that may bring them near Earth and to publish these data openly and internationally.