Achievements (Halo 3)

There are a total of 79 achievements that can be unlocked in Halo 3. 49 were shipped with the game, worth a total of 1000, with another 30 downloadable content achievements worth. Some achievements unlock certain armor permutations.

Downloadable Content
An additional 30 achievements, totaling 750 GS, were leaked at PAX 08. All 30 of these, worth of 750 GS, were added in Title Update 2.

* Bungie later announced in a weekly update that, to receive this achievement, you actually have to reach 7 EXP on the 7th of any month, not enter with 7 EXP.

Trivia

 * Unlocking at least 49 achievements and 1000 Gamerscore gives the Security helmet and Katana body in multiplayer. These pieces may relock unless all 49 non-DLC achievements have been unlocked. Finding all skulls unlocks the Hayabusa Armor.
 * Out of the original 49 achievements, the online-only achievements (except maybe next time buddy and MVP) were only obtainable in the playlists Lone Wolves and Head to Head. Occasionally Double EXP Weekend playlists did have Ranked FFA playlist(s) as well, such as  Legendary Brawl. The newer achievements added with AU2 are obtainable in both ranked and social playlists, providing it is on the required map(s).
 * Some players have been known to use the language glitch for the Halo 3 online achievements, and this is considered boosting and frowned upon by most Halo gamers.
 * The UNSC Spartan Achievement, which you receive for reaching sergeant status online, has been known not to unlock for some people regardless of rank. This was later investigated by Bungie and was found that this is because their consoles are turned off before actually receiving the achievement. It should also be noted that, with the new EXP system, the achievement can be obtained through reaching UNSC Spartan level on a playlist. This still, unfortunately, will not unlock the UNSC Spartan pin on a player's global service record.
 * The "Vidmaster Challenges" were from Bungie's Marathon series, where players who wished to prove they were the absolute best had to complete insane challenges such as playing through the game without the use of health items, with only fists, on the highest difficulty setting. Those that succeeded made videos to mark their achievement, thus spawning the term "Vidmaster". When all Vidmaster Challenges are completed then Recon armor is unlocked in Halo 3 multiplayer.