Currency
It's time to track down Riel. No, that isn't a typo; the Riel is Cambodia's national currency. However, the currency you need bring with you is US Dollars. Riels are typically used for modest transactions under a few dollars, with dollars being utilized elsewhere. Specifically, you'll be paying in US Dollars for everything that costs more than $1.
You'll be given Riel for any change less than $1. When you withdraw money from a Cambodian ATM, it will be in US Dollars or, in some situations, you may be given the option of choosing between US and Riel. Always use US because it is more commonly accepted and easier to grasp when converting. You'll also have a lot fewer notes in your wallet, making things considerably easier all around.
In Cambodia, little change rules, so don't expect to pay for your tuk tuk with a $100 or even a $20 bill. Also, keep in mind that torn notes, especially those of greater value, will not be accepted by many establishments. Wrinkled $100 bills are frequently rejected.