The Kepler mission is "specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near thehabitable zone and determine how many of the billions of stars in our galaxy have such planets." Kepler's only instrument is a photometer that continuously monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data is analyzed to detect periodic fluctuations that indicate the presence of extrasolar planets that are in the process of crossing the face of other stars.

Kepler is a project under NASA's Discovery Program of relatively low-cost, focused science missions. The Ames Research Center is the home organization of the science principal investigator and is responsible for the ground system development, mission operations and science data analysis. The development of the Kepler mission was managed by NASA's Jet Propulsion Laboratory until December 2009, but was then transferred to the Ames Research Center. Ball Aerospace & Technologies Corp. was responsible for developing the Kepler flight system.

The Kepler observatory is currently in active operation, with the first main results announced on 4 January 2010. As expected, the initial discoveries were all short-period planets, with longer period planets expected later. The first six weeks of data revealed five previously unknown planets, all very close to their stars. Among the notable results are one of the least dense planets yet found, two low-mass white dwarf stars that were initially reported as being members of a new class of stellar objects, and a well-characterized planet orbiting a binary star.

On 2 February 2011, the Kepler team announced the results from the data of May to September 2009. They found 1,235 planetary candidates circling 997 host stars, more than twice the number of currently known exoplanets. TheKepler results included 68 planetary candidates of Earth-like size and 54 planetary candidates in the habitable zone of their star, with 5 exoplanets falling in both categories. The team estimated that 5.4% of all stars host Earth-size planet candidates and 17% of all stars have multiple planets. As the mission continued, additional longer-period candidates were found – as of September 2011, there are a total of 1,781 candidates.

As of November 2011, the Kepler mission has released data from the first three "quarters" of its operation (roughly three months each, excluding the first quarter which is approximately one month). Additionally, the data used to support each discovery has become public following the formal publication of papers announcing the discovery.

The Kepler team originally promised to release data within one year of observations. However, this plan was changed after launch, with data being scheduled for release up to three years after its collection. This resulted in considerable criticism, leading the Kepler science team to release the second and third quarters of their data one year and nine months after collection. At the Extreme Solar Systems II meeting, Kepler Deputy PI Dr. Natalie Batalha announced that the fourth quarter of data (collected between December 2009 and March 2010) would be made publicly available in December 2011, and the data through June 2010 would be made public in January 2012.