Mariner 10, the first dual-planet, gravity-assist mission, was launched by an Atlas/Centaur Mariner launch vehicle from the National Aeronautics and Space Administration—Kennedy Space Center in Cape Canaveral, Florida on 3 November 1973. Shortly after liftoff, a series of earth and Moon observations were made. These were followed by the initial trajectory correction maneuver and a period of interplanetary cruise operations. An additional trajectory correction maneuver was made several weeks prior to the encounter with Venus to refine the flyby on 5 February 1974 to 5000 km (3000 miles) above the surface of the planet. Extensive scientific observations of Venus took place over a period of about one week. Several thousand TV images were transmitted to Earth, many of which showed spectacular ultraviolet cloud formations and motions. The post-Venus trajectory required only a modest correction to place the spacecraft on a flight path that passed within the planned 1000 km (620 miles) of the surface of Mercury on 19 March 1974. Extensive TV imaging, together with other scientific observations, provided the first in-depth information concerning Mercury. The Mariner 10 mission is described, including engineering highlights of the flight and the key scientific results. The post-Mercury operation plan is discussed, the initial results of the second encounter with Mercury are given, and the possibilities of a third encounter are presented.