Campaign mode is the story driven gamemode of almost all Halo games, where potentially multiple players take control of one or more playable characters at a time, through the canonical story of the Halo Universe.
In Halo: Combat Evolved, Halo 3, and Halo 4, the player plays as the Master Chief Petty Officer John-117, a SPARTAN-II. In Halo: Reach, the player takes the role of Noble Six, a SPARTAN-III whose name and past are unknown. In Halo: Spartan Strike, players take control of an unknown SPARTAN-IV.
In Halo 2, the player plays as both John-117 and the Arbiter Thel 'Vadam(ee), a Covenant Sangheili, which allows the player a more personal view of Covenant society. In Halo 3: ODST, the player plays as a variety of Orbital Drop Shock Troopers within the squad, Alpha-Nine. In Halo: Spartan Assault, the player takes control of Spartan Sarah Palmer and Spartan Edward Davis. In Halo 5: Guardians, the player plays as both John-117 and Spartan Jameson Locke, a SPARTAN-IV who works with the Office of Naval Intelligence.
In Halo: Combat Evolved and Halo 2, player two is a double of John-117 or Thel 'Vadam. The same applies to Halo 4, however this extends to all four players being the same as player one.
In Halo 3, player two plays as Thel 'Vadam, player three is N'tho 'Sraom, and player four is Usze 'Taham. In Halo: Reach, each player plays as their customized Noble Six. In Halo 5: Guardians, players two through four play as either the surviving members of Blue Team or Fireteam Osiris, depending on the section of the game.
In Halo Wars, the campaign has a different gaming aspect. The game is a strategy video game, and the player takes control of a base, multiple troops and several vehicles. It is up to the player to decide where a squad will go or attack, when a building will be constructed, etc. Again, the game follows the canonical storyline of the Halo Universe. In co-op, the second player gets control of the same resource pile and bases as player one.
Halo: Spartan Strike currently has the most levels as it features five "operations" with six levels each adding up to a total of 30 total levels. Halo: Spartan Assault features the second most as its campaign features five "operations," each with five levels, adding up to a total of 25 levels. Halo 2, Halo Wars and Halo 5: Guardians are all tied for the third-longest campaigns (all three have 15 levels total). Halo: Reach is fourth with 11 levels. Halo 3, Halo 4, Halo: Combat Evolved, and Halo 3: ODST are all tied for fifth with 10 levels each.
In Halo 2's campaign, the player plays as John-117 for 8 (7 if one does not count The Armory) levels, while the player plays as Thel 'Vadam(ee) for 7 (6 if one does not count The Heretic) levels. In Halo 5: Guardian's campaign, the player plays as John-117 and Blue Team for 3 levels and Jameson Locke and Fireteam Osiris for 12 levels.
All Halo campaigns feature cinematics. Halo 2's, Halo 3's, Halo 3: ODST's, Halo: Reach's, and Halo 4's first levels are all cinematics. Halo 4 and, in The Master Chief Collection, Halo 3 feature their last levels as cinematics.
The cinematics in Halo Wars, Halo Wars 2, Halo 2: Anniversary are created by Blur Studios. The Prologue and Epilogue in Halo 4 and the opening and closing cinematics of Halo 5: Guardians are also rendered by Blur. The cinematics in Halo: Spartan Assault and Halo: Spartan Strike were created by Sequence Group. All other cinematics in all other games are rendered in-game.
Campaign can be played on four difficulty levels: Legendary, Heroic, Normal, and Easy. Games past Halo: Combat Evolved have an unofficial difficulty known as the Mythic difficulty, which is Legendary with all Skulls activated. Officially, it is known as LASO (Legendary, All Skulls On) and has been named such in achievements and challenges.
- Main article: Non-Playable Character
Each Halo game feature a variety of AIs. The AI's skill and health are determined by Difficulty and Skulls. Allied AIs are allowed to trade weapons with players and drive them around in certain situations. Enemy AIs can hijack player vehicles and use them against them.
- Main article: Cooperative Play
In addition, Halo: Combat Evolved and Halo 2 have a co-op campaign option, permitting two players to play through the campaign either in split-screen or via system link between two Xbox gaming consoles. For Halo 3, Halo: Reach, and Halo 4, the number of players permitted has changed to allow up to four players and the use of Xbox Live. Halo: Reach as well, has Matchmaking support for the Campaign where instead of inviting or joining other players, it features the ability to search for other players. Campaign Matchmaking was made available to the public as of October 15, 2010. Halo 5: Guardians has removed split-screen and system link functionality.
In Halo: Combat Evolved Anniversary and Halo 2: Anniversary, the player can switch between the original graphics of that game and the newly remade graphics. As well, this switch changes all the audio, and certain animations.
- Main article: Campaign Scoring
In Halo 3, Halo 3: ODST, Halo: Reach, Halo: The Master Chief Collection, and Halo 5: Guardians there is the ability to turn on campaign scoring. In campaign scoring, players earn points and medals for killing enemies, getting assists and destroying vehicles.
- Main article: Terminal
In Campaign Mode, Terminals can be found hidden away. They can provide secret information about events, filling the player in on Halo's backstory.
In Halo: Combat Evolved Anniversary's terminals are in video form, and track 343 Guilty Spark before, during, and after John-117 lands on Installation 04, and the events of Halo: Combat Evolved. In Halo 4, the terminals are again in video form and follow the story between the Librarian and the Ur-Didact. Both video sets can be viewed on Halo Waypoint.
In Halo 2: Anniversary, the terminals are in video form and follow multiple characters including the Arbiter Thel 'Vadam, Sesa 'Refumee and 343 Guilty Spark, and an unnamed San'Shyuum scribe. These terminals are accessible on the Halo Channel.
The equivalent of terminals also can be found in multiple games. In Halo: Reach, Data pads give the story of the Assembly. In Halo 3: ODST, Audio logs give information through audio and some video about the story of Sadie Endesha as the Covenant attack the city of New Mombasa. In Halo Wars, there are collectible Black Boxes that add information to an in game timeline. Halo 5: Guardians has Mission Intel, which are audio logs that give insight into a variety of different storylines and characters.
- Main article: Skull
In Halo: Combat Evolved, Halo 2, Halo 3, Halo Wars, and Halo 5: Guardians. skulls can be collected. These skulls when picked up, can unlock various modifiers that will change gameplay in some way. In all other games, these skulls are automatically unlocked.
- Halo: Combat Evolved/Halo: Combat Evolved Anniversary - Levels | Cinematics
- Halo 2/Halo 2: Vista/Halo 2: Anniversary - Levels | Cinematics
- Halo 3 - Levels | Cinematics
- Halo Wars - Levels | Cinematics
- Halo 3: ODST - Levels | Cinematics
- Halo: Reach - Levels | Cinematics
- Halo 4 - Levels | Cinematics
- Halo 5: Guardians - Levels | Cinematics
- Halo Wars 2 - Levels | Cinematics
- Halo: Combat Evolved and Halo 4 start with John-117 leaving a cryo pod and Halo 3 ends with him entering one.
- Halo 3 and Halo 5: Guardians are the only games where campaign co-op mode can be considered canon, since its previous games features 2 John-117s or Arbiters, and there are only 6 members in Alpha-Nine and Noble Team.
- In Halo: Combat Evolved, Halo 3, and Halo Wars, 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 08 firing (and getting destroyed by the effects, as well as possibly the Ark), and Halo Wars ends with a Shield World imploding.
- Every Halo game apart from Halo: Reach starts and ends with the player in space. In Halo: Combat Evolved, the game starts with John-117 on the UNSC 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 in 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 closing of the cryotube of the late Sergeant Forge. Halo 4 starts with John-117 aboard the stranded Forward Unto Dawn and ends with him aboard the UNSC Infinity.
- Halo 3 and Halo: Combat Evolved both end the same way with you blowing up Installation 04, a Warthog Run, and then end up drifting in Space.
- When playing campaign of FPS Halo games on high difficulty levels, Speed Running can be used to complete levels quickly.