US dollars are as commonly used as the Cambodian Riel and even Thai Baht is acceptable in many places.Most hotels and many restaurants and shops set their prices in dollars. Small transactions are usually done in Riel. Always carry some small Riel for motorcycle taxis, snacks, beggars and other small purchases.
Riel notes come in 50, 100, 200, 500, 1000, 5000, 10,000, 50,000 and 100,000 denominations, but the distinctive red 500 Riel note is the most commonly used.
Credit cards and travelers checks are not common but are catching on. US dollar travelers checks are much more easily encashed than any other kind.
Money changers cluster around the markets. When accepting money, inspect the bills. Marred Riel is acceptable tender, but the tiniest tear in a large US note renders it worthless.
There are banks in all of the larger provincial capitals, including Phnom Penh, Siem Reap, Sihanoukville, Battambang. Banks can change money, effect telegraphic transfers and some banks can cash travelers checks and accept Visa cards.
There is only two ATM in Cambodia, at the Canadia Bank in Phnom Penh and ANZ Royal Bank in Phnom Penh, and you must have a local account in order to use it. You cannot access foreign accounts from this.