As the name suggests, Airborne Warning And Control System aircraft provide an overview of the aerial situation in a combat area.
The Sentry is a modified Boeing 707/320B airliner with a rotating radar dome held 14 feet (4.2 metres) above the rear fuselage by two struts. The dome is 30 ft (9.1 m) in diameter, 6 ft (1.8 m) thick.
It contains a radar system that can detect, identify and track enemy aircraft and direct fighters to meet them, from the ground up into the stratosphere.
The growing capabilities of the fighters themselves have meant that E-3s are now of more use in co-ordinating large numbers of aircraft.
The Sentry can also provide information for ground-attack aircraft and relay the situation across a battlefield back to commanders.
Most early versions have been upgraded with better computers, more radios, secure communications, maritime surveillance equipment and five extra radar display consoles.
The US Air Force has 33 E-3s but they are also in use within Nato - which has lent five of its 17 E-3s to help to cover the continental United States since the 11 September terrorist attacks.
The UK has seven, France four and Saudi Arabia five.
There might be more than 20 people on board each plane: Four flight crew plus 13 to 19 specialists, depending on the mission being flown.
The radar has a range of more than 250 miles (375.5 km) for low-flying targets - more for those at higher altitudes.
But it uses old technology in radar terms: Its slowly rotating antenna means the position of each target it is tracking is updated only every 10 seconds or so.
Newer, "phased-array" radars can provide a continuous target position.
Also, the terrain it is flying over limits the range of its sensors. They worked superbly in the flat landscape of southern Iraq, but Afghanistan's mountains were a different matter.
E-3 Sentry (Awacs)
Radar dome with a range of 250 miles, and electronic surveillance equipment