I attempt to identify the central conceptual and methodological challenges that must be overcome if the risk assessment of terrorism is to make the same progress that in recent years has distinguished the risk assessment of other forms of violence. Four principal conclusions are offered. First, clarity from the outset on what is being assessed — the risk of terrorism in the aggregate, or of specific types of terrorism, or of specific phases in the process of becoming a terrorist, or of specific roles in terrorist activity — is a prerequisite to progress in research.
Second, one current approach to the risk assessment of more common violence (e.g., assault) — the approach known as structured professional judgment — usefully may be applied to the risk assessment of terrorism. However, given that many known risk factors for common violence are in fact not risk factors for violent terrorism, the substantive content of any instrument to assess the risk of terrorism will be very different from the substantive content of current instruments that address common violence. Third, since there is little existing evidence supporting the non-trivial validity of any individual risk factors for terrorism, the highest priority for research should be the identification of robust individual risk factors. Promising candidates include ideologies, affiliations, grievances, and “moral” emotions. Finally, it is highly unlikely that an instrument to assess the risk of terrorism can be validated prospectively. An infrastructure for facilitating access to known groups of terrorists and non-terrorists from the same populations may be crucial for conducting a program of scientifically rigorous and operationally relevant research on the individual risk assessment of terrorism.