This chapter focuses on the nature, scope, and legitimacy of the use of force by UN peacekeeping operations within the framework of international law. Before clarifying the legal authority of UN peacekeepers to use force, it considers the historical and conceptual foundations and development of the use of force in UN peacekeeping. It then outlines the normative framework for use of force, including the categorization and legal bases for use of force under international law, and its relation to the jus ad bellum. The chapter also discusses the ‘basic principles’ of UN peacekeeping, namely consent of the main parties to the conflict, impartiality, and non-use of force except in self-defence, along with the goals of protecting civilians and responding to violations of international human rights law. Finally, it analyses the operational and practical challenges that arise due to the legal problems resulting from the use of force by UN peacekeepers.