This thesis examines the experience of the United Nations Transitional Administration for Eastern Slavonia, Baranja, and Western Sirmium (UNTAES), which was set up in 1996 with the task of administering the North-Eastern region of Croatia during its transition from rebel-Serb control to Croatian sovereign control. The thesis provides an assessment of UNTAES from the perspective of a developmental approach to peacebuilding, by examining UNTAES's record in three functional areas - employment, education, and housing and property - as examples of its more general approach to socio-economic policy. In addition to concentrating on the ways in which UNTAES addressed the regulation and reintegration of these policy areas within the Croatian system, and hence on the period covering the duration of its mandate (1996-1998), the effects of the mission's policy in the immediate post-reintegration phase up to 2000 are also considered. The thesis draws primarily on evidence collected through fieldwork in the territory formerly administered by UNTAES, including key documents concerning the reintegration of Eastern Slavonia, and interviews with participants and other individuals with close knowledge of the mission. Development is increasingly recognised in scholarly and policy literature as a crucial element for achieving sustainable peace. Although the mission's mandate reflected this trend, the thesis argues that UNTAES was reluctant to interpret its developmental tasks in a comprehensive way, in particular when this involved initiating potentially destabilising and expensive policies. Rather, UNTAES's approach was dominated by short-term imperatives and expedience to the detriment of long-term processes, such as reconciliation and the establishment of an integrated multiethnic society. The conclusions drawn are that such a course of action has proved inadequate for building sustainable peace and the success of this mission can, in this respect, be questioned. Furthermore, the experience of UNTAES makes clear that development and peacebuilding, although increasingly integrated in peacebuilding theory and in UN rhetoric, in practice, still operate as largely distinct fields.