What is THAAD?
The Terminal High Altitude Area Defence (THAAD) missile system is designed to counter and shoot medium and intermediate range ballistic missiles.
THAAD's powerful radar allows for missiles to be detected and destroyed before they are launched. With a range of up to 200 kilometres and the ability to reach altitudes of 150 kilometres, it is specifically designed to counter a possible missile attack from North Korea.
Lockheed Martin, the security and aerospace company that was the main contractor for the system said it is highly mobile and is comprised of interceptors, launchers, a radar, a fire control unit, and support equipment.
THAAD in South Korea became operational on Tuesday.