In this thesis I examine the ways in which analysing open-air preaching as ‘radical street performance’ can inform our understanding of this expression of Christian preaching. Open-air preaching is commonly associated with negative stereotypes. Most contemporary homiletical writers also largely neglect considering this practice. Through my research, I posit radical street performance as a constructive and illuminating way to understand and analyse open-air preaching. In chapter 1, I introduce the practice of open-air preaching in relation to relevant homiletical literature. In so doing, I challenge the commonly held stereotypes about open-air preaching. I do so with reference to the long and diverse nature of the practice. In chapter 2, I critically analyse existing ‘preaching as performance’ literature. I first demonstrate the ways in which these authors show the suitability of performance as a concept for understanding preaching. I then go on to consider the limitations of their understandings of preaching as performance for exploring open-air preaching in performance terms. I do this to establish the immediate theoretical context for my own research. In chapter 3, I develop this argument further drawing on the work of performance theorists Jan Cohen-Cruz and Baz Kershaw. I argue accordingly, that radical street performance is a valuable way of understanding and analysing open-air preaching as performance. On the basis of these theoretical and methodological foundations, in chapters 4-6, I explore three case studies of open-air preaching according to this analytical approach. In chapter 4, I focus on the late eighteenth and early nineteenth century evangelical preaching of James Haldane (1768-1851), whose open-air preaching was directly related to his move to congregational Independency. In chapter 5, I explore the early to mid twentieth century open-air preaching of George MacLeod (1895- 1991), founder of the Iona Community. In chapter 6, I analyse the open-air preaching of OAC Ministries GB, a contemporary organisation that seeks to promote and practice open-air preaching in a creative way. The outcomes of the original research in chapters 4, 5, and 6 demonstrate the applicability and versatility of radical street performance as a way of understanding and analysing open-air preaching in performance terms. It also provides original understandings of the dynamics of each example of open-air preaching examined, highlighting differences and similarities between them. In chapter 7, I draw together by way of conclusions, the theoretical, theological, and practical outcomes of the research for the practice of open-air preaching and the consequent implications for in-church preaching. In this way I present open-air preaching as a minority but significant practice of incarnational witness which exists in a tensive relationship with the dominant practice of in-church preaching.