This volume explores the many ways in which conflicts between secular worldviews and religions shaped the history of the twentieth century. It introduces the notion of 'apologetics' to highlight a common feature of these conflicts: both secular and religious groups employed a mixture of learned argument and popular propaganda to defend their faith, but also to come up with new forms of outreach and mission. By bringing the dynamics of religious and secular apologetics into a comparative perspective, and drawing on examples from Western Europe, the USSR, the USA, North Africa and Asia from the 1920s to the present, the volume offers important historical perspectives on current debates over the place of religion in contemporary politics.