This article studies the relationship between sustainability and institutional memory in postwar Croatia. Is institutional memory preserved after interventions end? Is so, how and by whom? What are the causes of loss of institutional memory? What are the consequences for sustainability and accountability? When international organizations pulled out of peacebuilding operations in Croatia in the mid to late 2000s, they quickly lost their institutional memory of their projects. Donors, international nongovernmental organizations, and intergovernmental organizations all lost their ability to recall the work they had done in the past. Using interview, ethnographic data, and archival documents gathered over five years (2008–2013), I define three types of memory—archival, human, and electronic—and show how each of these forms of memory eroded as international projects in Croatia ended. The loss of international memory has implications for international organizations' own stated goals of sustainability and their ability to achieve and assess sustainability, and for the downward accountability of donors to their beneficiaries and the countries they worked in.