A new type of right-wing populist parties has established itself in the party systems of Western and now also Central Eastern Europe over the past 20 years. Some have entered government and have been able to influence the political agenda. As a result of this development, populism has spread to mainstream parties, which have adapted in content and style. What are the causes of populist protest and the tendency towards a plebiscitary transformation of political systems? Is populism an ideology or merely a political style that can be combined with any content? How have established political forces reacted? Do populists pose a threat to democracy, or are they a legitimate and useful early warning system? These questions are examined from a general theoretical perspective and within the framework of comparative country contributions.