This thesis examines one of the most remarkable and poorly understood features of contemporary International Relations: the world-wide rise of so-called 'fundamentalist' religions. Despite the earlier predictions of many social scientists, the globalization of modern thought-forms, institutions and technologies has not resulted in the 'withering' of religion and its irrevocable expulsion from the public domain. Instead, religious 'fundamentalism' has in recent times emerged as a potent political force in both the 'modernised' countries of the North and the 'modernising' countries of the South. To shed light on the nature, causes and repercussions of this surprising development in world politics the thesis undertakes a detailed analysis of both the concept of 'fundamentalism' and the reasons that have been given for the global growth of 'fundamentalist' movements. The thesis then proceeds to an in-depth examination of the rise of Brazilian Evangelicalism. Although Evangelical Protestantism has existed in Brazil for almost 200 years, only in the last 50 years has it grown extremely rapidly and gained large numbers of converts within this traditionally Catholic country. The thesis examines these developments - together with various explanations that have been put forward to account for them - and concludes that the explosive growth of Brazilian Evangelicalism is closely related to the negative impact that modernity has had upon the country. Although the 'modernisation' of Brazil has led to rapid industrial development and ensured very high living standards for a wealthy elite, the majority of Brazilians have had to cope with deficient welfare services and poor employment prospects, whilst many Brazilians have been denied access to basic material necessities. In such circumstances of widespread social deprivation, the Evangelical churches have attracted many Brazilians - particularly from amongst the poorer classes - by providing them with a range of benefits, including a sense of personal empowerment and worth, supportive social relationships, opportunities for emotional and spiritual expression, and the prospect of a better life. The thesis therefore argues that the popularity of Brazilian Evangelicalism is linked to the negative consequences of modernity and questions the commonly-held view that religious 'fundamentalisms' are essentially reactionary movements seeking to defend a traditional way of life from the secularising forces of modernity.