In the Ph.D. curriculum, there are five clusters (principal fields). Each of the clusters represents an area of substantial faculty research interest in the School, in some cases cutting across disciplinary lines. Individual faculty choose which clusters, if any, to join; depending on interests, some faculty may wish to join more than one cluster. The faculty coordinator of each cluster assumes primary responsibility for students in that cluster.

The clusters that are open to new enrollments are Security Studies, and Science, Technology, and Environmental Policy.