The official currency in Cambodia is the Riel which come in denominations of 100, 200, 500, 1000, 2000, 5000, 10000, 50000 and 100000 notes. However, in the major towns and cities, US dollars can be freely spent though travelers are recommended to use smaller denomination notes as change may be difficult. Most places will refuse old, tatty or damaged US bills. Due to the economic problems in the region, the currency has slipped from 2500 Riels to 3700-3900 per one US dollars (at the time of this writing), meaning Cambodia is an even better value for money tourist destination than before. Popular local and international currencies can be exchanged openly and freely. Major credit cards are only accepted in a few places so traveler cheques or cash are recommended.

In Cambodia, there are no ATM machines, thus getting cash from your credit card is nearly impossible. Travelers may cash advances from their credit card at some shops, but the latter will charge high handling fees.

Traveler Cheques (TC)  - You can exchange TC at any bank in Cambodia, but you have to pay about 2-4% extra for converting it into US$ bill. 

As an advice, you should carry USD cash (with $20 and $100 notes) in addition to traveler cheques and credit card. Do not exchange all of your cash into the local currency at one time, but gradually. It is very difficult to exchange back to foreign currency - practically impossible for ordinary tourists!