Radical right parties have seen increasing electoral success throughout Europe. What does this imply for parties and party systems? Do established mainstream parties adjust their policy positions in response to successful radical right parties? If yes, is this “contagious effect” restricted to specific party families or is this an overall trend within European politics? Tarik Abou-Chadi and Werner Krause investigate these questions and find that mainstream parties adjust their policy strategies when confronted with a successful radical right challenger and shift toward more anti-immigrant positions. Using a novel research design, they can demonstrate that these shifts are not just a response to changing public opinion but can be causally attributed to the success of the radical right.