As of 15th of June 2017, EU roaming charges are dropped. This simply means that you can have a sim card from your operator in a country that is a member of European Union, and use your plan in other European countries without paying roaming fee.
Roaming in European Union
You can also use your data plan in other countries. However, there is fair use policy, which you should be careful of. There are several details as well to be aware of. Therefore, you better read all the specific rules before enjoying roaming free use of your mobile plan in your phone traveling around Europe.
This issue has been in the agenda of European Commission for a while. It is definitely great news to facilitate easy travel in Europe.
Changes in EU Roaming
You can find more information about the changes in European Union roaming charges on the press release of European Commission: