The Cambodian currency is the riel, but the U.S. dollar is more widely accepted, with many high-end businesses actually requiring payment in dollars. Thai baht are usually accepted in bordering provinces.
The official exchange rate is approximately 4,000 riel to one U.S. dollar and 100 riel to the Thai baht. It's possible to change dollars to riel just about anywhere. Banks and businesses usually charge 2% to cash traveler's checks.
ATMs are available in Phnom Penh, Siem Reap, and Sihanoukville, mostly at ANZ and Canadia banks, although there are numerous other banks starting to install them. Using an ATM will cost you $5 per withdrawal. Credit cards are accepted at major hotels, restaurants, and at some boutiques. Cambodian banking hours are shorter than in many Western countries, generally from 8 am until 3 or 4 pm. ATMs are available 24 hours.