Do U.S. travelers need return tickets in Europe? No. Return or onward tickets are not required for US travelers in Europe. However, you must be ready to explain your travel plans and how they fit within the immigration laws of your destination country. A travel plan that includes dates, hotel reservations and bus or train tickets can go a long way. Remember that upon entering a country, immigration officers have the right to question the nature of your travel as far as they see fit. If the pieces don’t add up, you may very well find yourself delayed fielding questions from immigration.
The golden rule here is to know the rules and follow them. That means know how much time the country grants tourists, know what documents you need to show and show a clear understanding of the importance of leaving the country on time. Lack of knowledge of immigration rules or a generally careless attitude may lead you to further questioning.
Remember that Europe is currently facing extreme immigration challenges and many countries are on high alert for suspicious travelers. As a U.S. traveler you may not face the same scrutiny as say a traveler arriving from Turkey, but it is up to you to explain your travel plan and why you are not a threat to break local immigration (or other) laws.