Missing In Action, better known as MIA, is a casualty classification used by the United Nations Space Command that refers to a member of the UNSC who has been reported missing following a combat mission and whose status as to injury, capture, or death is unknown. The missing combatant must not have been otherwise accounted for as either killed in action or a prisoner of war.
When SPARTAN-IIs are killed, they are reported as Missing or Wounded in Action to create the illusion of SPARTAN Immortality. This apparently helped boost morale for the UNSC. SPARTAN-IIIs are not normally subject to this, as the very existence of the program remains classified. However, at least two were listed as such in accordance with the tradition by Kurt-051.
- Randall-037, James-005, Soren-066 and all the Spartans aboard the UNSC Spirit of Fire (Alice-130, Jerome-092, Douglas-042), those trapped in the Onyx Shield World (Fred-104, Kelly-087, Linda-058, Tom-B292, Lucy-B091, Ash-G099, Olivia-G291, Mark), Team Katana, Joshua-029, John-117, Gray Team, and Team X-ray are the only Spartans who truly became missing.
- Before his discovery on Requiem by the UNSC Infinity, John-117 was considered Missing in Action due to the fact that there was no body recovered and no evidence that he had actually died, despite Terrence Hood's quote, "Hard to believe he's dead." Although Thel 'Vadam does say "Were it so easy", suggesting he knows John survived.
- During the video log in the level 343 Guilty Spark, the automated video recorder regards Wallace A. Jenkins as possibly MIA, due to the termination of his recording as a result of his infection by the Flood.
- Soren-066 is the only crippled Spartan to be MIA.