The rise of populism has promoted a broad, vivid and flourishing debate in the social sciences that seems to have arisen even in the face of the ties between right-wing populism and the extreme right. The social sciences are struggling with how properly to conceptualise and theorise populism as a social and political phenomenon. Incongruity or asynchrony of events in factual history and their being conceptualised is obviously critical with regard to the problems that arise with defining and conceptualising populism. The plurality of usages, applications and meanings of populism thus only shows how, in a vivid debate, scholars can observe a contest for coming to terms with a concept that remains in flux and that needs to be continually revised given rapidly changing social conditions. The chapter also presents an overview of the key concepts discussed in this book.