Concerns about how terrorists might attack in the future are central to the design of security efforts to protect both individual targets and the nation overall. In thinking about emerging threats, security planners are confronted by a panoply of possible future scenarios coming from sources ranging from the terrorists themselves to red-team brainstorming efforts to explore ways adversaries might attack in the future. This paper explores an approach to assessing emerging and/or novel threats and deciding whether, or how much, they should concern security planners by asking two questions: (1) Are some of the novel threats 'niche threats' that should be addressed within existing security efforts? (2) Which of the remaining threats are attackers most likely to execute successfully and should therefore be of greater concern for security planners? If threats can reasonably be considered niche threats, they can be prudently addressed in the context of existing security activities. If threats are unusual enough, suggest significant new vulnerabilities, or their probability or consequences means they cannot be considered lesser included cases within other threats, prioritizing them based on their ease of execution provides a guide for which threats merit the greatest concern and most security attention. This preserves the opportunity to learn from new threats yet prevents security planners from being pulled in many directions simultaneously by attempting to respond to every threat at once." --P. [4] of cover.