Efforts to understand the rivalry between Riyadh and Tehran have produced a body of literature that can be separated into three camps. The first suggests that the rivalry is best understood through a balance of power in the Gulf. The second suggests that religion plays a prominent role in shaping the nature of the rivalry and that so-called proxy conflicts have been drawn along sectarian lines. The third suggests that a more nuanced approach is needed, drawing upon concerns about regime power and legitimacy – externally and internally – with instrumentalised use of religious difference. This chapter introduces the broader parameters of the debate around the Iran–Saudi rivalry, incorporating key works in the field to date. It also provides a historical contextualisation of this key geopolitical relationship. This introductory chapter concludes by outlining the individual chapter contributions to the volume.