This study contains the results of research on best practices in nationbuilding. It is intended to complement a companion volume, America’s Role in Nation-Building: From Germany to Iraq, which focuses on U.S.-led nationbuilding efforts. Its purpose is to analyze United Nations military, political, humanitarian, and economic activities in post-conflict situations since World War II, determine key principles for success, and draw implications for future nation-building missions. The study contains the lessons learned from eight UN cases: Belgian Congo, Namibia, El Salvador, Cambodia, Mozambique, Eastern Slavonia, Sierra Leone, and East Timor. It also examines the nationbuilding effort in Iraq.