Currency and ATMs
Cambodia has its own currency, the Cambodian riel, but uses the US dollar alongside this at an exchange rate of 4,000 riel to 1 USD. Riel can’t be bought or changed outside of Cambodia, so the best thing is to take US dollars (in banknotes only though: coins are not used in Cambodia). ATMs in Cambodia generally dispense US dollars. Dollars and riel are used interchangeably; it’s possible to pay using dollars, riel or a mix of both.
It's best to break large notes in a big shop or supermarket, and keep some small notes handy for tuk tuks and roadside vendors who won't carry much change. Also, be aware that some shops or vendors will not accept a note in less-than-perfect condition.
General Budget for Cambodia
It’s possible to travel very cheaply in Cambodia by staying in budget accommodation and eating like a local; the cost of living is very low. However, those with a larger budget will certainly find a way of spending it.