Deep Impact, or at least part of the flight system, is designed to crash into comet 9P/Tempel 1. This bold mission design enables cometary researchers to peer into the cometary nucleus, analyzing the excavated material with its imagers and spectrometers. The book describes the mission, its objectives, expected results, payload, and data products in articles written by those most closely involved. This mission has the potential of revolutionizing our understanding of the cometary nucleus.