Credit

The credit, abbreviated as "Cr" or "cR," is the standard currency of the UNSC. It is the only currency used by the UNSC that has yet been mentioned, and it is unclear if it is based off any older money systems. As of 2552, cR is still in use. Credits can be spent in the form of "credit chips," which seem to be the equivalent of the modern credit card.

Credit Farming
In Halo: Reach, there are many strategies to earning many credits to increase in rank and so on. A very popular technique is shown below.
 * A common trick is to go to the 3rd mission, and sprint past all of the enemies until you get to the room with the target locator. Fire at the large group of covenant that has probably amassed in chasing you, and move to the back of the blast doors, where you will get a checkpoint. if you have done this correctly, you should get a Killionare medal each time. Keep reloading the last checkpoint as many times as you want, then finish the rest of the level.

Trivia

 * UNSC citizens sometimes casually refer to credits as "credaroos," analogous to the way Americans refer to dollars as "buckaroos."
 * There is no known Covenant monetary unit that can be compared to cR and the UNSC has no idea how much money their weapons would cost.
 * In the trailer of Halo 3: ODST, there are monitors that are intended to announce information to citizens. One of the messages says "Your tax dollars at work." As dollar is no longer functioning currency, the sign should really read "tax credits," but it is likely that the old phrase has not changed since the 21st century.
 * According to Halo: The Fall of Reach, there are no coins or papers used in the credit system, but in the Audio Logs in Halo 3: ODST, an old woman is heard breaking an ATM with money in the form of coins and presumably paper coming out. Another ATM, found in gameplay, which also houses an Audio Log, is also constantly spitting out money until the Audio Log is collected. It is unknown whether this is an oversight or if the currency is different than credits.
 * Credits are used by the player in Halo: Reach to buy Armor Permutations and Armor Effects for their Spartan character in the game. These credits also determine Rank.
 * If a Marine is killed with friendly fire in Halo 3, another one may say "Okay, now you owe me twenty credits!"
 * The headhunters Jonah and Roland would constantly bet twenty credits while on missions.
 * They aren't used in Forge. Instead, the USD ($) is used.
 * It could be presumed the trade for credits is $1=4cR. This is reinfoced by the fact the a person gets four credits per normal kill, and how soldiers bet 20 credits instead of the stereotypical 5 dollars.

Internal

 * Halo: Reach
 * Credit Bounty

External

 * Bungie.net's review of the Civilian Warthog