Comets consist of the most primitive, i.e. unprocessed, material in the solar system. To understand the earliest epoch of solar system formation, we must investigate the nature of this material. Much of what we know of comets presently comes from remote sensing using Earth-based telescopes, plus a few brief flybys that have added only a few hours of in-situ observation. The Rosetta mission is poised to make a dramatic advance in our understanding of comets by matching trajectories with comet 67P/Churyumov-Gerasimenko, orbiting it, and setting a lander on its surface. This volume describes the scientific objectives of the mission, the expected cometary environment, and the instruments that will probe the comet.