Gemini 8 (officially Gemini VIII) was the sixth manned spaceflight in NASA's Gemini program. The mission conducted the first docking of two spacecraft in orbit, but suffered the first critical in-space system failure of a U.S. spacecraft which threatened the lives of the astronauts and required immediate abort of the mission. The crew was returned to Earth safely. The only other time this happened was on the flight of Apollo13.

It was the twelfth manned American flight and the twenty-second manned spaceflight of all time (including X-15 flights over 100 kilometres (62 mi)). Command pilot Neil Armstrong's flight marked the second time a U.S. civilian flew into space (Joseph Albert Walker became the first US civilian on X-15 Flight 90). Armstrong had resigned his commission in the United States Naval Reserve in 1960. The Soviet Union had launched the first civilian, Valentina Tereshkova (also the first woman) aboard Vostok 6 on June 16, 1963.

Gemini VIII was planned to be a three-day mission. After being launched into an 87-by-146-nautical-mile (161 by 270 km) orbit, on the fourth revolution it was to rendezvous and dock with an Agena target vehicle, which had been earlier launched into a 161-nautical-mile (298 km) circular orbit. This was to be the first space docking in history. Four separate dockings were planned.

During the first docking, Pilot David Scott planned to perform an ambitious, two-hour-and-10-minute extra-vehicular activity (EVA), which would have been the first since Ed White's June 1965 spacewalk on Gemini IV. On a 25-foot (7.6 m) tether for one and a half revolutions around the Earth, Scott would have retrieved a nuclear emulsion radiation experiment from the front of the Gemini's spacecraft adapter, then activate a micrometeoroid experiment on the Agena. Then he was to move back to the Gemini and test a minimum-reaction power tool by loosening and tightening bolts on a work panel.

During the EVA, after Armstrong undocked from the Agena, Scott was to don and test an Extravehicular Support Pack (ESP) stored at the back of the spacecraft adapter. This was a backpack with a self-contained oxygen supply, extra Freon propellant for his Hand Held Maneuvering Unit, and a 75-foot (23 m) extension to his tether. He would practice several maneuvers in formation with the Gemini and Agena vehicles (separated at distances up to 60 feet (18 m), in concert with Armstrong in the Gemini. Scott never got to perform this EVA, due to the abort of the flight because of an emergency which occurred shortly after docking.

The flight also carried an additional three scientific, four technological, and one medical experiment.