Since the early 1990s, building peace in war torn societies has emerged as a new field of international practice. Given its ad hoc mode of development, this new field of practice has been characterised by considerable concern with improvement and collective learning. These efforts notwithstanding, the field continues to attract criticisms about its failure to engage with local contexts and to address the political dimensions of peace processes. Such criticisms are particularly important since many peacebuilding actors and observers have repeatedly emphasized the political nature of peace processes. Not only is depoliticization criticized by members and observers of the field alike, but it has also been linked to drawbacks in particular interventions. Why, then, has peacebuilding emerged as a depoliticized and decontextualized practice in the first place, and how can we account for the persistence of depoliticization? The thesis focuses on the agencies tasked with conducting peacebuilding. It argues that the emergence and persistence of depoliticization can only be understood by analysing the mismatch between actors' institutional identities and practices and the particular nature of the challenges they face. Most peacebuilding agencies originate from the peacekeeping, humanitarian, and development communities and are committed to observing neutrality and/or impartiality. They initially became involved in peacebuilding on the premise that their task consisted in 'assisting' the implementation of a peace agreement. However, as peacebuilding often involves a significant degree of ongoing political contestation, most newly minted peacebuilding agencies have been caught between their commitment to neutrality and/or impartiality and the inherently political nature of peacebuilding and peace processes. The thesis combines recent strands of practice theory with a Bourdieusian field perspective to emphasize the dynamic link between actors' identities and practices. The framework I advance not only allows for understanding the reproduction, but in particular the emergence and transformation of international practices. To this end, it analyses the practices of a number of 'key players' within the emerging peacebuilding field across two peacebuilding cases (Bosnia and Herzegovina and Timor Leste). Overall, the thesis argues that the new practice and related thinking on peace and conflict in international organizations continue to be shaped by the 'foundational dilemma' and in particular by the ways in which peacebuilders have tried to resolve it while preserving their institutional identities.