In 1969, an influential volume arising from a conference at LSE began with a declaration phrased to echo Marx and Engels: ‘A spectre is haunting the world: the spectre of populism’ [1]. More than half a century has gone by, and the warning re-mains timely. Donald Trump may be gone from the White House, but populism is still a powerful force in world politics. From Mexico City to Manila and Mumbai, from Budapest to Brasilia and Buenos Aires, and from Ankara to certain party offices in Amsterdam and Athens, Warsaw and Washington, both the right-wing and the left-wing varieties of populism are alive and kicking. Once an intensely contested concept, the meaning of ‘populism’ has recently stabilised. Jan Werner Müller defines populism as ‘a particular moralistic imagination of politics, a way of perceiving the political world that sets a morally pure and fully unified ... people against elites who are deemed cor-rupt or in some other way morally inferior [2].’ Very much along the same lines, according to Mudde populism is ‘first and foremost, a set of ideas focused on a fundamental opposition between the people and the elite’ and arguing for implement-ing something like a ‘general will’ of the people [3]. Framed in this way the populist label can apply to social movements, parties or political leaders. Recent years have witnessed an upsurge in populist phenomena in many countries, raising ques-tions about how they should be understood. Are the causes of populism economic or cultural? National or local? Is populism a threat to liberal democracy? If so, what kind of threat? And what can be done about it? This book brings together authors from a range of disciplinary perspectives, employing a variety of meth-ods, to tackle these thorny issues.