The Halo main theme is set in the key of D major, with two sharps: F and C. The first part of the song is a choir chant which is repeated at the end, and this is set to a beat of 4 4, with the middle section featuring string instruments and percussion; this is set to a beat of 12 8 and is significantly faster than the choir chant. The starting choir's tempo is a beat per minute of 95, while the middle section speeds up to a beat per minute of 120, before the ending choir chant which settles back to 95.
In its original incarnation, featured frequently in Halo: Combat Evolved, it placed heavy emphasis on deep, powerful percussion instruments and fast paced strings. The song is heard most prominently at the start of The Silent Cartographer, during the beach landing and during the end credits. The choir chant at the beginning and end of the piece was used for the main menu screen in-game. 343 Guilty Spark will hum it in Halo 3 with the IWHBYD skull on. It is the first true music heard by the player. On the Halo: Original Soundtrack, however, it was the last track, and also included the bonus track Siege of Madrigal at the end. It also plays in The Maw after Echo-419 gets shot by two Covenant Banshees.
For Halo 2, the song was remixed into the MJOLNIR Mix. The basics of the song remained the same, but it featured electric guitar overlays by former Whitesnake guitarist Steve Vai. Halo 2 continued to use the song in its original form in certain parts, such as at the beginning of the level Metropolis. The MJOLNIR mix was played at the end of Metropolis, as John-117 boards the Scarab, and later during the end credits, is the first track on the Halo 2 Soundtrack Volume 1.
The theme was again revised for Halo 3, this time recorded with a live orchestra instead of synthesized strings. The version used in Halo 3 finished with the ending used at the end of The Maw, as opposed to the reprise of the opening chant that the original theme and the MJOLNIR mix had used.
A partial remix of the track appears in the Halo Legends Original Soundtrack. This remix plays during Halo Legends episode Origins, during the scenes depicting the escalation of the Human-Covenant war.
The Halo Theme did return in Halo 4, part of the chant is heard at the beginning of the level Composer, but Neil Davidge, who wrote most of the music for Halo 4, did not include the Halo Theme as the title music. The crescendoing section from the Halo Theme can also be heard in the track 117 as well as in the track Sacrifice both composed by Kazuma Jinnouchi.
In the Halo: The Master Chief Collection Announcement Trailer, a modified version of the song was heard, playing for a short while when John-117's visor was first shown.
- Halo was composed over the course of three days in the summer of 1999 by Martin O'Donnell.
- On October 6, 2012, the Ohio State University Marching Band performed a video game medley showcase in which they formed multiple shapes and configurations that referred to popular games. Most of these games were Nintendo series such as Pokémon and Mario. However, during the middle of the performance the announcer made a tribute to Halo, and the band formed the classic logo while playing the main theme.
- During the Halo 4 level Composer, the Halo chant can be heard very faintly in the background when Installation 03 appears.