This essay is an enquiry into the context, nature, and significance of militant sectarian conflict in Pakistan. The parties to the conflict are the Sunnis, who constitute the majority of Muslims in Pakistan, and the Shi‘a, a small but influential minority. Conflict between these two religious communities has deep roots in the history of Islam and of South Asia. In Pakistan, which aspires to be in some sense an ‘Islamic state’, sectarian conflict is part of, and interacts with, broader issues concerning the place of Islam in public life. This essay seeks to analyse some of factors which have contributed, especially in the past twenty-five years or so, to militant sectarian conflict in Pakistan and to assess the significance of this rather neglected form of Islamic radicalism.