Tipping culture in Europe is very different, so there are some habits American travelers will want to leave at home. For most, tips are not expected in Europe, and it's fine not to tip at all, especially in more casual settings like bars or cafes. In other cases, though, small tips may be warranted.
Can you tip in U.S. dollars in Europe?
The US dollar is not the medium of exchange. Most likely, the individual would have to locate a reliable cash exchange source, which might not always be easily available. If you want to provide a tip, be kind and respectful. When at your European designation, withdraw cash using your ATM card and offer tips in euros.