Having decided to abolish money altogether between 1975 and 1980 when the Khmer Rouge took over, Cambodia afterwards reintroduced the riel. Coins worth 50, 100, 200 and 500 riel are in circulation but are rarely used. The notes begin with a 50 which is also not often seen, the 100 much more popular. Notes worth 1000, 2000 and 5000 riel are the most commonly used but there are 50,000 and even 100,000-riel notes. Expect to use US dollars and even Thai baht much more regularly.
Currency exchange is no great problem in Siem Reap given that there are places to change money at the airport; nearly everyone accepts US dollars and Thai baht and in fact, both are usually preferred. There are numerous exchange kiosks in the centre of Siem Reap; and while many hotels are also happy to exchange money, their rates are likely to be inferior. No places should charge fees for changing money. Try to pay for small purchases with riel and larger ones with foreign currency; otherwise you are likely to get Cambodia's multi-currency system working against you.
Cash machines are readily available in the city; so too places that will offer cash advances on your credit card, usually for a flat fee so do it in large amounts. Visa and MasterCard are preferred. All mid- and top-range hotels and restaurants will usually accept credit cards, as will tour operators and some souvenir shops. Many other places will not, so make sure to hold onto plenty of cash.