This introductory chapter traces the history of the modern interfaith movement, from its early beginnings in the United States in the 19th century, to its emergence and development around the globe over the course of the 20th century, and in particular in the wake of the events of 9/11. We sketch an overview of the wide-ranging initiatives around the world that fall under the banner of ‘interfaith’, and outline the movement’s shifting agenda(s). In doing so, we raise important questions about what it means to consider such disparate efforts to constitute a social movement. A central claim of this introduction, and indeed the volume more generally, is that the interfaith movement is best understood not simply as a ‘religious’, but also a socio-political phenomenon. We trace the historical divergence between social movement theory and religion, and underline how the interfaith movement presents an interesting challenge, and indeed a valuable opportunity, to theorists on either side of the disciplinary divide.