Although it was only designed for a ninety-day surface mission, the Viking 1 lander ultimately transmitted science messages to Earth for seven years. This authoritative history chronicles the remarkable achievements of the Viking program during its first three decades. Commissioned by NASA, it recounts the events surrounding the first planetary landing on Earth's closest neighbor and the first on-site search for evidence of extraterrestrial life. It also portrays a human drama in which thousands of professionals from government, industry, and academia joined together to accomplish the seemingly impossible. This history begins with a survey of the qualities that make the surface and atmosphere of Mars prime targets for scientific investigation. A retrospective of NASA's Mariner program follows, detailing the series of robotic interplanetary probes that led to the initiation of the Viking program in 1968. The authors trace the ensuing technological developments, including the first lander vehicles and orbiter. They also profile the cooperation of managerial, technical, and scientific teams during the mission's data-gathering and analysis phases. The final chapters outline the scientific results of the Viking investigations, examine some of the unresolved questions, and consider possible future explorations.