NASA's Kepler Mission is designed to determine the frequency of Earth-size and larger planets in the habitable zone of solar-like stars. It uses transit photometry from space to determine planet size relative to its star and orbital period. From these measurements, and those of complementary ground-based observations of planet-hosting stars, and from Kepler's third law, the actual size of the planet, its position relative to the habitable zone, and the presence of other planets can be deduced. The Kepler photometer is designed around a 0.95 m aperture wide field-of-view (FOV) Schmidt type telescope with a large array of CCD detectors to continuously monitor 100,000 stars in a single FOV for four years. To detect terrestrial planets, the photometer uses differential relative photometry to obtain a precision of 20 ppm for 12th magnitude stars. The combination of the number of stars that must be monitored to get a statistically significant estimate of the frequency of Earth-size planets, the size of Earth with respect to the Sun, the minimum number of photoelectrons required to recognize the transit signal while maintaining a low false-alarm rate, and the areal density of target stars of differing brightness are all critical to the photometer design.