Launching in 2008, Kepler will explore the structure and diversity of planetary systems, with a special emphasis on the detection of Earth-size planets. It will survey the extended solar neighborhood to detect and characterize hundreds of terrestrial and larger planets in or near the "habitable zone," defined by scientists as the distance from a star where liquid water can exist on a planet's surface. The results will yield a broad understanding of planetary formation, the frequency of formation, the structure of individual planetary systems, and the generic characteristics of stars with terrestrial planets.