A visitor entering the foyer of the Imam Al-Khoei Islamic Centre in New York, with its crystal chandelier and frieze of gilded verses from the Qur'an, will also certainly notice the framed portrait of an ageing, white-bearded, black-turbaned Shi'i cleric. During the research conducted for this book, I have encountered the same picture many times in many places: the library of a theological college in the Iranian seminaries of Mashhad, an orphanage in Beirut, a small religious school in Bangkok, the prayer room of a community centre in Paris, the website of a charitable association operating in India and the cover page of an Arabic magazine published in London. The face of Grand Ayatullah Abu al-Qasim al-Khu'i adorns the walls of places he never went to. From his classroom in the holy city of Najaf in Iraq, his teachings influenced millions of Shi'i followers across the world. An eminent scholar, spiritual leader and philanthropist, his legacy traverses time and borders.

Any considered assessment of Shi'i Islam requires looking with a transnational lens beyond the national framework. For centuries, religious networks defined by common affinities have been sustained across localities by the movement of peoples, the exchange of ideas and communal practices. The ‘ulama’ (sing. ‘alim; religious scholar) have come to represent the quintessential transnational actor and their continued visibility in the worldwide geography of Shi'ism is the focus of this book. In the sixteenth and seventeenth centuries, clerics from today's Lebanon, Bahrain and Iraq settled in Persia at the invitation of Safavid rulers who were converting the country from Sunnism to Shi'ism. Scholarly migration also reinforced the development of major centres of learning in the Iraqi and Iranian shrine cities where the most prominent religious scholars have offered guidance to believers worldwide. It was in the Iraqi seminaries of Najaf that the Persian Muhammad Husayn al-Na'ini designed his influential treatise in support of constitutionalism at the height of Iran's first revolution (1906–11) and that, decades later, the Iraqi Muhammad Baqir al-Sadr produced writings which inspired a new constitution for the Islamic Republic established after the country's second revolution (1978–9).