Harvard Model United Nations | 6,127 followers on LinkedIn. Harvard Model United Nations (HMUN) is the oldest, largest, and most prestigious United Nations simulation for high school students held in the United States. HMUN is a four-day international relations simulation for high school students held annually in downtown Boston. At HMUN, delegates gain insight into the workings of the United Nations and the dynamics of international relations by assuming the roles of UN representatives and members of other international bodies and national cabinets.