Campaign is the single player aspect of Halo: Combat Evolved, Halo 2, Halo 3, Halo 3: ODST, Halo: Reach, and Halo 4 where the player takes control of a single character at a time and plays through the events of the Halo storyline. It also refers to the single player aspect of Halo Wars, where the player takes control of an entire UNSC Marine Corps force.
In Halo: Combat Evolved, Halo 2, Halo 3, and Halo 4, the player plays as the Master Chief Petty Officer John-117, one of the last remaining SPARTAN-IIs. In Halo 2, the player plays as both Master Chief and the Arbiter Thel 'Vadam, a Covenant Elite, which allows the player a more in-depth view of Covenant society. In Halo 3: ODST, the player plays as "The Rookie" an Orbital Drop Shock Trooper, and takes on the roles of other ODSTs from the squad during flashbacks. In Halo: Reach, the player plays Noble Six, a SPARTAN-III whose name, gender, and past are unknown.
In Halo Wars, the campaign has a different gaming aspect. The game is a real-time strategy game, and the player takes control of entire bases, troops and vehicles. The player decides where a squad will go, what type of building will be constructed, etc. Again, the game follows the canonical storyline of the Halo Universe.
In any game, the Campaign Mode can be played on four difficulty levels, listed in order from most difficult to the easiest: Legendary, Heroic, Normal, and Easy. In addition, Halo: Combat Evolved and Halo 2 have a co-op Campaign option permitting two players to play through campaign either in split-screen or via system link between two Xbox gaming consoles. For Halo 3 and Halo: Reach, the number of players permitted has increased to up to four players. The use of Xbox Live for co-op has also been added. Halo: Reach also features matchmaking support for campaign where players can be automatically matched to each other. 
- Halo 3 is the only game where campaign co-op mode is considered to be canon, since all other games feature multiple numbers of the playable character. In Halo 3, the second player plays as the Arbiter, the third as N'tho 'Sraom and the forth as Usze ‘Taham.
- All games except Halo: CE also have a unique, unofficial difficulty known to fans: Mythic, which is Legendary with all Skulls activated. In Halo: Reach, weekly challenges even feature completing levels on this difficulty. This is also known as LASO (Legendary, all skulls on).
- In Halo: Combat Evolved, Halo 3, Halo Wars and Halo 4, the endings of the three games end with a huge Forerunner structure getting destroyed: Halo: Combat Evolved ends with Halo Installation 04 being destroyed, Halo 3 ends with Halo Installation 04B firing (and getting destroyed by the effects), Halo Wars ends with a Shield World imploding and Halo 4 ends with the Didact's ship, Mantle's Approach and the Composer destroyed.
- Every Halo game apart from Halo: Reach starts and ends with the player in space. In Halo: Combat Evolved, the game starts with Master Chief on the Pillar of Autumn and ends with him in a Longsword. Halo 2 starts with him on Cairo Station and ends with him on the Forerunner Dreadnought. Halo 3 starts off with him crashing towards Earth and ends with him stranded inside the UNSC Forward Unto Dawn. Halo 3: ODST starts with The Rookie on the UNSC Say My Name and ends with him on an ONI orbital facility. Halo Wars starts with Captain Cutter on the UNSC Spirit of Fire talking about the war on Harvest and ends with the freezing of Professor Anders. Halo 4 starts with the Master Chief inside the Forward Unto Dawn and ends inside the UNSC Infinity.
- AI cannot perform needle supercombines, except in Halo: Combat Evolved and Halo 4.