The range of activities undertaken by multilateral peace operations has significantly increased over the last 20 years, an important aspect of this is the detention of individuals. However, for a long time, it remained unclear what rules and laws apply to detentions in peacekeeping operations. Only recently have rule of law principles emerged within the United Nations (UN) and among implementing states. Hirschmann’s chapter demonstrates how accountability dynamics contributed to a growing rule of law for detentions. It shows how standard setting, monitoring and sanctioning by regional organizations, non-governmental organizations and international courts contributed to core rule of law elements, namely the UN Secretary-General’s Bulletin on the Observance of International Humanitarian Law, specific provisions for the operation in Kosovo and the Copenhagen Process on the handling of detainees.