When you pay by card abroad, the card machine or ATM may ask whether you want to pay in U.S. dollars or in the local currency. Choose the local currency, such as euros, lira or shekels.
If you choose dollars, the shop’s payment company converts the price for you at its own exchange rate, with a markup built in. If you choose the local currency, your own card converts it at the Visa or Mastercard rate, which is almost always cheaper. The same applies at ATMs: if the screen offers to convert your withdrawal into dollars, decline it.
If a cashier charges you in dollars without asking, you can ask them to run the payment again in the local currency. For the lowest cost overall, travel with a credit card that has no foreign transaction fee.