This chapter examines the nature of religion and fundamentalism and their relationship to politics. It first defines religion before discussing the nature and extent of religiosity worldwide. It then considers whether religion can be regarded as an ideology and goes on to assess its relationship with secular ideologies. It also explores arguments about the role of religion in politics, focusing on the secular state and ‘religious talk’ in the political sphere. Finally, it reviews the concept of fundamentalism as a form of political belief, the nature of religious fundamentalism, and the impact of movements based on religious fundamentalism in the modern world. These issues are illustrated with case studies relating to Christian (Protestant) fundamentalism, religious identity in the United Kingdom, the relationship between politics and religion in the United Kingdom vs. the United States, whether the faithful have a religious duty to get involved in politics, and Islamism.