How is Foreign Exchange handled When Traveling Outside the US?

We are planning a two week vacation to Europe in the Spring. However, we are concerned about how the foreign exchange will be done before we leave the US. Is this something that needs to be done before we go or is it taken care of when we arrive at the airport? If the airport does handle the exchange rate of money, will it be before our flight to Europe leaves or after we arrive there? We really need to know the way this will be done. This will be our first time traveling to Europe and if it is successful, we plan on doing this every year.