Kepler is the tenth mission launched under NASA's Discovery Program and NASA's first mission dedicated to the search for planets beyond our solar system, with the goal of determining the frequency of Earth-sized planets within the habitable zones of Sun-like stars. Extrasolar planets are detected using the transit method, in which the passage of a planet across the disk of a star produces minute (10−2–10−5) reductions in the amount of light from that star. Kepler monitors the brightness of stars using a space-based wide-field telescope coupled to a precision photometer operating in a single broad optical bandpass. Approximately 160,000 main-sequence stars make up the primary observing program. Several thousand additional sources are normally observed during each observing season; these sources comprise the asteroseismology, guest observer, and other science targets. Using data from its first 16 months of operation, Kepler has identified more than 2300 exoplanet candidates, validated on the order of 40 single- and multiple-planet systems, discovered the first circumbinary planets, discovered a super-Earth in the habitable zone of a Sun-like star, and made fundamental advances in the fields of stellar structure and variability.