IntroductIonThe use and utility of military force have been central, if sometimes underlying and unarticulated, themes in discussions about the purposes, practices, and, indeed, the very identity of United Nations peacekeeping since its inception. The precise historical and normative context within which those discussions have taken place has necessarily evolved over time. And yet, as the 2015 report of the High-Level Independent Panel on Peace Operations (HIPPO) makes clear, many of the key questions raised by the use of force in peacekeeping-be they of a practical or conceptual kind-are not fundamentally new (UN 2015). Chief among these is an overarching question that also frames and animates the present chapter, to wit, what are the limits to the use of force in UN peacekeeping? In approaching this question, the chapter and the arguments it advances have been divided into three closely connected parts. 1