Peacekeeping is widely considered to be an effective means of preventing civil war recurrence. However, as peace has collapsed in a considerable number of cases despite peacekeeping efforts, we are left with the question which combinations of peacekeeping environments and peacekeeping missions lead to a lasting peace. This chapter compares 22 peacekeeping missions between 1990 and 2012. While prominent United Nations documents assume that the success of post-conflict peacekeeping primarily depends on the features of the mission itself, the analysis shows that characteristics of terminated civil war have a strong influence on whether peace endures. Restrained peacekeeping, defined by low troop density, non-robustness, and a lead nation that is not a permanent member of the Security Council, only succeeds in preserving peace in conducive environments. Inconclusive war endings, evenly distributed military capabilities at war’s end, ethnic conflicts, and high intensity create a particularly difficult context for peacekeeping.