This introduction offers a more comprehensive overview of the debate surrounding the radicalisation of the mainstream. The recent electoral success of far-right parties and movements has highlighted the shift of previously marginal ideologies toward the political centre. In a number of European liberal democracies, ideas characterised by antagonism towards minorities, gender and sexual diversity, and immigrants appear to be gaining increasing acceptance as common sense. Islamophobia, in particular, has become a potent unifying force among radical-right actors. The increasing acceptance of such discourses poses a significant challenge to democratic values and necessitates scholarly scrutiny. Whilst a substantial body of research has been conducted which contrasts radical actors with the democratic mainstream, the heterogeneity of the latter—and its role in this process—remains insufficiently explored. This volume addresses this gap by offering new empirical insights and proposing an agenda to investigate how mainstream institutions, media, and everyday practices absorb, adapt to, or resist far-right ideas. By means of comparative case studies and interdisciplinary approaches, the contributions examine the subtle dynamics that enable exclusionary narratives to enter and transform democratic discourse. This volume thus invites scholars and policymakers to reconsider the process of radicalisation—not only at the political margins but also in relation to the political and cultural mainstream. Furthermore, it prompts a reflective process concerning the potential for prevention or counteraction of this development.