Extra-vehicular activity (EVA) is work done by an astronaut away from the Earth and outside of his or her spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth (a spacewalk) but also applies to an EVA made on the surface of the Moon (a moonwalk). In the later lunar landing missions the command module pilot did an EVA to retrieve film canisters on the return trip.

Due to the different designs of the early spacecraft, the American and Soviet space programs also define an EVA differently. Russians define an EVA as occurring when a cosmonaut is in a vacuum. An American astronaut, in contrast, is not considered to have made an EVA until at least his head is outside the spacecraft. The term stand-up EVA (SEVA) is used for being partly outside.

EVAs may be either tethered (the astronaut is connected to the spacecraft, oxygen can be supplied through a tube, no propulsion is needed to return to the spacecraft) or untethered. When the tether performs life support functions such as providing oxygen, it is called an umbilical. For untethered EVAs during space flight, capability of returning to the spacecraft is essential; see Manned Maneuvering Unit (MMU).

An EVA is dangerous for a number of different reasons. The primary one is collision with space debris. Orbital velocity at 300 km above the Earth (typical for a Space Shuttle mission) is 7.7 km/s. This is 10 times the speed of a bullet, so the kinetic energy of a small particle with a mass 1/100th that of a bullet (e.g. a fleck of paint or a grain of sand) is equal to that of a bullet. Every space mission creates more orbiting debris, so this problem will continue to worsen (see also Kessler Syndrome).

Another reason for danger is that external environments in space are harder to simulate before the mission, though approximate simulations can be achieved at facilities like NASA's Neutral Buoyancy Laboratory. Space walks are avoided for routine tasks because of their danger. As a result the EVAs are often planned late in the project development when problems are discovered, or sometimes even during an operational mission. The exceptional danger involved in EVAs inevitably leads to emotional pressures on astronauts.

Other possible problems include a spacewalker becoming separated from their craft or suffering a spacesuit puncture which would depressurize the suit, causing anoxia and rapid death if the spacewalker is not brought into a pressurized spacecraft quickly.

One astronaut has suffered a spacesuit puncture. During STS-37, a small rod punctured the glove of one of the astronauts (the name is undisclosed, but it was either Jerry L. Ross or Jay Apt). However, the puncturing object, which stabbed the astronaut's hand as well, held in place, resulting in no detectable depressurization. In fact, the puncture was not noticed until after the spacewalkers were safely back inside Atlantis. *

Aleksei Leonov's EVA did not pass smoothly, although this was not reported at the time. Reportedly his spacesuit was overinflated due to vacuum conditions and so had to be vented, in order to return through the airlock. Because Leonov was breathing an "Earth-normal" mixture, the nitrogen in his blood briefly bubbled, resulting in a case of "the bends", normally suffered by deep-sea divers when they ascend to the surface too fast.

As of 2006, no catastrophic incident has ever occurred during an extra-vehicular activity, and no astronaut or cosmonaut has ever died during one. Still, some scientists are developing tele-operated robots for outside construction work, to potentially eliminate the need for human EVAs.

NASA exhaustively researched and tested all equipment to be used during EVA. Included in this research was a wrist timepiece that would withstand the extremes of the outer space environment. After purchasing samples of the top branded chronographs (i.e. Rolex, Omega, Longines, etc) and putting them through lab testing (including extreme cold/hot temperatures, humidity, G-forces, etc.), only the Omega Speedmaster model passed the NASA tests. It became the only approved watch for EVA during the Gemini project, was the first watch worn on the moon• and continues to be the only watch that may be worn during EVA.