Over the last three decades, numerous radical right populist parties (RRPP) have emerged, developed, and strengthened their electoral weight in Western Europe. Yet, while several RRPP have managed to formally participate in government coalitions (such as in Italy, Austria, and Switzerland) or to informally support minority governments (such as in Denmark, and in The Netherlands) and while other RRPP have become highly visible opposition forces (such as in France, and Germany), the influence exercised by RRPP remain underexplored. It is essential to focus on their policy influence because of their electoral strength but also because they are often perceived by journalists, citizens, policy-makers as well as by researchers as a threat to democracy. As a reaction, mainstream parties tend to adopt specific strategies - such as measures of militant democracy towards RRPP. The aim of this book is to contribute to theoretical and empirical research in political science by bringing together a variety of contributions about the influence of RRPP in terms of policies on their core issues. To that end, we ask under which circumstances these parties are able to do so in contemporary Western Europe. This book proposes to focus on the role played by party status. Are RRPP better able to leave their imprints when they are in power or support minority governments than when they hold opposition or outsider status in Western Europe?