It is generally recognized that the impact of a 6-to-10-km-diameter (4-to-6-mi) asteroid 65 million years ago contributed significantly to the extinction of the dinosaurs and many other species living at the time. Fortunately, these types of events are very rare, but there is growing recognition that impacts of smaller objects can cause significant regional disasters and that these events can happen relatively frequently. Humanity may now have the technology to divert these smaller objects, but we need to know where they are and to decide how and when to take action. This article overviews the nature of the threat, describes options for deflecting a threatening object, and discusses uncertainties that will affect the decision to act.