The modern history of Saudi–Iranian relations (dating back to the formation of the Saudi state in 1932) can be characterized broadly into five distinct phases. The first is between 1932 and 1979, which is characterized by regional distrust yet a willingness for the two states to engage with each other. The second is the period after the revolution until three years after the end of the Iran–Iraq War – where a catastrophic earthquake provided an opportunity to reset relations – which was driven by existential concerns about the nature of political organization and competition over Islamic legitimacy. The ensuing period from 1991 to 2003 was one in which a burgeoning rapprochement began to unfold, driven by domestic factors across the Gulf. The fourth period ran roughly from 2003 to 2011, in which the bombastic nationalism of Mahmoud Ahmadinejad, the Iranian president (2005–13) ran up against the belligerent ‘Axis of Evil’ narrative within the US War on Terror. The fifth period emerged after the Arab Uprisings in 2011, providing the two states with opportunities to exert influence across the Middle East through the provision of support to groups across the region. The events of the Arab Uprisings provided further opportunities to increase their influence, particularly in those states where regime-society relations began to fragment. In this chapter I offer a brief genealogy of relations between Saudi Arabia and Iran with a focus on the construction of competing nomoi – visions of regional order – which play out across the transnational field and resonate within states.