HELMETS
* Mark VI: Unlocked by default.
* CQB: Unlocked by default.
* EVA: Complete Campaign on Normal or higher difficulty.
* EOD: Complete Campaign on Legendary difficulty.
* Hayabusa: Collect all 13 different Skulls in Campaign.
* Security: Earn 1000 gamerscore in total from Halo 3 achievements.
* ODST: Complete the Spartan Graduate achievement. (In the new rank progression system) - Reach the rank of Private in any playlist ranking.
* Mark V: Complete the UNSC Spartan achievement. (In the new rank progression system) - Reach the rank of Sergeant in any playlist ranking.
* Rogue: Complete the Spartan Officer achievement. (In the new rank progression system) - Reach the rank of Lieutenant in any playlist ranking.
* Recon: One way is to complete all of the Vidmaster challenges in Halo 3 and Halo 3: ODST. The armor can also be unlocked by achieving something that makes Bungie feel you deserve it, such as a Forge map, film or screenshot or winning against Bungie in a Bungie VS World event.
* Scout: Complete the Used Car Salesman achievement
Shoulder Pieces
* Mark VI: Unlocked by default.
* CQB: Unlocked by default.
* EVA: Complete The Ark in Campaign on Normal or higher difficulty.
* EOD: Complete the level The Ark on Legendary difficulty.
* Hayabusa: Collect at least 9 skulls in Campaign.
* Security: Earn 35 Achievements or higher.
* Scout: Complete the Mongoose Mowdown achievement.
* Recon: Get all the Vidmaster Achievements in Halo 3 and Halo 3: ODST. It can also be unlocked by doing something that makes Bungie feel you deserve it, such as a Forge map, film, or screenshot, or participating in a Bungie vs. World Event.
Chest Armor
* Mark VI: Unlocked by default.
* CQB: Unlocked by default.
* EVA: Complete The Tsavo Highway on Normal difficulty or
higher.
* EOD: Complete The Tsavo Highway on Legendary difficulty.
* Hayabusa: Collect at least 5 skulls in Campaign.
* Katana (Add-On): Unlock at least 49 achievements out of 79 and have at least 1000 Gamerscore in Halo 3. NOTE: There are reports from people who get it with less achievements. Since the release of the mythic map pack the Katana seems to unlock for those that download early(without the need for 1000 gamerscore OR 49 achievements)
* Scout: Unlock the Too Close to the Sun Achievement, although has been reported that it may appear later sometimes.
* Recon: Get all the Vidmaster Achievements in Halo 3 and Halo 3: ODST. It can also be unlocked by doing something that makes Bungie feel you deserve it, such as a forge map or screenshot and also winning a contest or various competitions that Bungie composes.
ELITE ARMOUR
HELMET
* Combat: Unlocked by default.
* Assault: Unlocked by default.
* Flight: Unlock either Campaign Complete: Heroic or Campaign Complete: Legendary Achievements.
* Ascetic: Unlock the Steppin' Razor achievement.
* Commando: Unlock the Overkill Achievement.
Shoulder Pieces
* Combat: Unlocked by default.
* Assault: Unlocked by default.
* Flight: Complete the level The Ark on Heroic or Legendary difficulties.
* Ascetic: Unlock the Overkill achievement.
* Commando: Automatically unlocks for everyone after Title Update 1.
Chest Armor
* Combat: Unlocked by default.
* Assault: Unlocked by default.
* Flight: Complete the level Tsavo Highway on Heroic or Legendary difficulties.
* Ascetic: Unlock the Up Close and Personal achievement.
* Commando: Unlock the Triple Kill achievement.