This thesis examines the challenges that accusations of heresy pose for peacebuilding processes. While heresy is often perceived as a general factor in intractable conflict, contributing to impasses between minority and majority religious groups, little is known about its impact on peacebuilding efforts. This study advances the understanding of the complex realities of conflicts involving claims of heresy by examining (1) how some groups are socially constructed as heretical and specifically how that contributes to conflict intractability, and (2) the special challenges posed by claims of heresy to peacebuilding efforts. To investigate these issues, this thesis examines the experiences of the internally displaced Ahmadiyya in Lombok and Shia from Sampang. Drawing on fieldwork conducted in Indonesia, the thesis explains the ongoing challenges heresy claims pose in these cases and suggests some potential paths forward for peacebuilding in Indonesia. In 2002, the Ahmadiyya community settlement in East Lombok, West Nusa Tenggara was set on fire by a mob, leaving buildings burned to the ground, people dead and injured, and forcing the rest of the community to flee from their homes. Driving the violence was the belief that the Ahmadis - people who followed Ahmadiyya teachings - were heretics because their teaching deviated from Sunnism, practised by most Indonesian Muslims. Leaving East Lombok, the Ahmadis faced more violence and rejection. After they were expelled from several other villages and had no place to live, the local government in 2006 decided to house the Ahmadiyya in an internally displaced persons (IDP) camp; almost twenty years later, they remain there. Theirs is not a unique story. In a similar turn of events in 2012, the Shia community from Sampang, Madura, East Java, had to flee their hometown after their settlement was attacked and burned. As with the attacks on the Ahmadi communities, this violence was also driven by differences in religious beliefs held by the Shia minority and Sunni majority in Sampang. Like the Ahmadis, the Shia of Sampang were also eventually relocated to an IDP shelter, where they continue to live. As this thesis demonstrates, accusations of heresy levelled against the Ahmadiyya and Shia populations of Lombok and Sampang drive this conflict and hamper efforts to resolve it. Heresy is a conflict between religious groups or within a group and, as this thesis demonstrates, is socially constructed and fundamentally political. As the cases of Ahmadiyya and Shia highlight, the idea that enemies within are more threatening than external enemies - which is central to the designation by religious and sometimes government authorities of certain people as heretics - leaves no room for compromise or acceptance of differences and this, in turn, contributes to conflict intractability. For this reason, existing efforts to resolve these conflicts by reconverting those accused of heresy to the Sunni Islam practised by the majority of Indonesian Muslims have failed. Drawing on a multidimensional framework for understanding how different aspects of religion contribute to conflict and to peace, this thesis identifies the ideas, institutions, and actors that have contributed to the ongoing Ahmadiyya and Shia conflicts. It argues, however, that just as religious institutions play a significant role in constructing teachings that are considered threatening - by defining which beliefs are orthodox, heterodox, and heretical - so, too, their construction of heresy has implications for peacebuilding. It also argues that, while religious figures at the grassroots can maintain divisions between orthodoxy, heterodoxy, and heresy, they also can serve as bridges to peace. In doing so, the thesis concludes that religious figures who are recognised by all parties to conflict as legitimate and authoritative actors have a vital role in peacebuilding, in their capacity to unite the Islamic community across and beyond the different sects.