The threat to Earth due to the impact of an Earth-crossing asteroid or comet is not new, but scientists have only recently recognized it as a continuing threat. The past ten years reveal several frightening near misses, and "hits" by bodies too small to survive long enough in the atmosphere to cause damage. The hazard created when an object strikes Earth varies greatly, from a minimum of no damage due to the object's destruction in the atmosphere to a maximum of millions of deaths, extinction of species, and an end to present civilization. While the potential hazard is great, the probability of occurrence is low, but an impact will happen; the only question is when. Technology exists to deal with this threat, but serious, organized effort to identify potential threatening objects and to deflect threatening objects is virtually nonexistent. This book analyzes planetary defense issues and develops seven recommendations for national policy and near term actions to be taken by the United States to develop and implement a system to defend Earth from potential impact by comets and asteroids.