ATMs are now nearly everywhere in Croatia except the most remote and uninhabited islands. Just look for the "bankomat" sign (see photo right).
Visa, Mastercard, Cirrus, Plus, Maestro, and Eurocard are the most commonly accepted cash cards; American Express is slightly scarcer. You will find ATMs at all major airports and train stations in Croatia and very near to the embarkation point for ferries and buses.
ATMs are convenient and reliable in Croatia, but ATM use for foreign travel has gotten expensive as banks and credit card companies cash in. The whirl of fees and charges has made it complicated to figure out exactly how much you're paying for the convenience of fast cash (compare ATM charges and fees for major US banks) which is why I recommend that you call your bank before leaving home and find out how much they're charging. You may be better off with a small, local bank rather than a big bank (big bank=big fees). While you're on the phone to your bank rep, see if you can avoid charges by switching to a business or preferred customer account.
Another money-saving tip is to avoid the option "pay in your home currency". The exchange rate is always unfavorable and you're still stuck with withdrawal fees.
Oh, and you'll need a four-digit PIN to withdraw money in Croatia.