The US missile defence programme is intended to be a defensive screen for the US and its allies.
Still in development, the finished system is designed to track and destroy incoming ballistic missiles fired by enemies from anywhere in the world.
The ground-based element targets long-range missiles in "mid-course". Others, including sea-based systems can attack shorter-range weapons in the boost (ascent) or terminal (descent) phase.
Infra-red detectors may locate some missiles from the heat created at launch, even before they clear cloud cover, when early-warning satellites and radars would be able to detect and track them.
Rapid development of space-based infra-red satellite systems mean early-warning procedures are improving, and feeding back the earliest possible trajectory estimates to the command centre.
High-resolution radars on the ground, designed to be able to discriminate accurately between closely-spaced objects, track the warhead and any decoys it launches.
The radar provides real-time continuous tracking data to the command centre, as well as data from earlier phases of a ballistic missile trajectory.
The command centre acts as the brains of the missile defence system, controlling and co-ordinating the whole operation.
On receiving information about an incoming missile, it communicates target information to one or more ground-based interceptors.
The ground-based interceptor (GBI) is the "weapon" of the missile defence system. Its mission is to intercept and destroy incoming ballistic missile warheads outside the Earth's atmosphere.
Each GBI site would be able to accommodate at least 10 interceptor missiles - and possibly as many as 100. GBIs in different countries may provide a second chance of a "hit" if the initial one is unsuccessful.
The interceptor missiles - or "kill vehicles" - use on-board sensors, as well as information from the ground, to isolate the warhead from the decoys and debris.
The kill vehicle uses small on-board rockets to manoeuvre and collide with the warhead, destroying both vehicles. It is equipped with a highly sensitive infrared seeker, and intercepts incoming missiles at supersonic speed.
Long-range missiles are so fast that there is a very short "window" in which interceptors may be effective. They are designed to travel at extremely high speeds to try to deal with incoming missiles that may exceed 420 metres (0.26 miles) per second in the mid-course phase.
By the time of descent, some missiles may be travelling at 13km per second (8 miles per second).