The debate on whether member states or bureaucrats have the upper hand in determining international organizations' behavior masks the complex reality in which states, officials, independent experts and civil society actors enter into discussions or negotiations about what is appropriate for a certain organization. In order to provide a more nuanced account of this reality, the dissertation applies the norm diffusion framework to explaining the evolution of aspects of UN peacekeeping policy and practice in the post-Cold War period. It looks at the policy norms prescribing missions to afford protection to civilians under imminent threat of physical violence and develop public information campaigns for the local population. The dissertation theorizes four diffusion mechanisms, which can operate in two modes, and five categories of scope conditions, which affect the functioning of each mechanism and mode. The study has contributed to the literatures on norm diffusion, international organizations, and UN peacekeeping.