Currency--The U.S. monetary system is painfully simple: The most common bills (all ugly, all green) are the $1 (colloquially, a "buck"), $5, $10, and $20 denominations. There are also $2 bills (seldom encountered), $50 bills, and $100 bills (the last two are usually not welcome as payment for small purchases). Note that redesigned $100, $50, and $20 bills were introduced in the last few years, but the old-style bills are still legal tender. Expect to see redesigned $10 and $5 notes by the time you arrive.
There are six denominations of coins: 1¢ (1 cent, or a penny); 5¢ (5 cents, or a nickel); 10¢ (10 cents, or a dime); 25¢ (25 cents, or a quarter); 50¢ (50 cents, or a half dollar); and the rare $1 piece (the older, large silver dollar and the newer, small Susan B. Anthony coin). In 2000, a new gold-toned $1 piece was introduced. 

Currency Exchange--The foreign-exchange bureaus so common in Europe are rare even at airports in the United States and nonexistent outside major cities. Youll find them in New Yorks prime tourist areas like Times Square, but expect to get extorted on the exchange rate.
American Express (tel. 800/AXP-TRIP; has many offices throughout the city, including at the New York Hilton, 1335 Sixth Ave., at 53rd Street (tel. 212/664-7798); the New York Marriott Marquis, 1535 Broadway, in the 8th-floor lobby (tel. 212/575-6580); on the mezzanine level at Macys Herald Square, 34th Street and Broadway (tel. 212/695-8075); and 65 Broadway, between Exchange Place and Rector Street (tel. 212/493-6500).
Thomas Cook Currency Services (tel. 800/223-9920; has locations throughout the city, including JFK Airport and 41 E. 42nd St. (tel. 212/883-0400). Call for additional locations.
Its best not to expect to change foreign money (or travelers checks denominated in a currency other than U.S. dollars) at a small-town bank, or even a bank branch in New York. In fact, its best to just leave any currency other than U.S. dollars at home--it may prove a greater nuisance to you than its worth.
Travelers Checks--Though travelers checks are widely accepted, make sure that theyre denominated in U.S. dollars, as foreign-currency checks are often difficult to exchange. The three travelers checks that are most widely recognized--and least likely to be denied--are Visa, American Express, and Thomas Cook/MasterCard. Be sure to record the numbers of the checks, and keep that information separately in case they get lost or stolen. Most businesses are pretty good about taking travelers checks, but youre better off cashing them in at a bank (in small amounts, of course) and paying in cash. Remember: Youll need identification, such as a drivers license or passport, to change a travelers check.
Credit Cards & ATMs--Credit cards are the most widely used form of payment in the United States: Visa (BarclayCard in Britain), MasterCard (Eurocard in Europe, Access in Britain, Chargex in Canada), American Express, Diners Club, Discover, and Carte Blanche; youll also find that New York vendors may accept international cards like enRoute, Eurocard, and JCB, but not as universally as AmEx, MasterCard, or Visa. There are, however, a handful of stores and restaurants that do not take credit cards, so be sure to ask in advance. And be aware that often businesses require a minimum purchase price, usually around $10 or $15, to use a credit card.
I strongly recommend that you bring at least one major credit card. Hotels, car-rental companies, and airlines usually require a credit-card imprint as a deposit against expenses, and in an emergency a credit card can be priceless.
Youll find automated teller machines (ATMs) on just about every block in Manhattan. Some ATMs will allow you to draw U.S. currency against your bank and credit cards. Check with your bank before leaving home, and remember that you will need your personal identification number (PIN) to do so. Most accept Visa, MasterCard, and American Express, as well as ATM cards from other U.S. banks. Expect to be charged up to $3 per transaction, however, if youre not using your own banks ATM.