Since the 1st of January 2002, the currency in Greece, being part of the European Community, is the EURO. This, of course, makes life much easier, especially for tourists coming from other European countries.
Bills are in denominations of 5, 10, 20, 50, 100, 200, 500 and coins are of 1, 2, 5, 10, 20, 50 Cents and 1 and 2 Euro.
In general try not to exchange money in the airport, rail or bus stations, in hotels, restaurants or stores, even if it might seem easier for you. You will get the best rates in banks, where the exchange rate does not vary significantly - but what can vary are the commission fees they charge.
Normally there is no charge for cashing traveler's checks.
The average exchange rates (Jan 2003) are:
USD: 1 : 1
English Pound: 1 : 0,642
YEN: 1 : 125