This article examines the scope and meaning of the principles of consent, neutrality/impartiality and self-defence in peacekeeping and claims that they play a constitutional role. More specifically, they distinguish peacekeeping from peace enforcement and allow the United Nations to play an active role in collective security. The author however claims that the demands of modern peacekeeping have put pressure on these principles and, therefore, the United Nations needs to reconsider their meaning in conjunction with its own principles and values.