Government organizations have put substantial effort into detecting and thwarting terrorist and insurgent attacks by observing suspicious behaviors of individuals at transportation checkpoints and elsewhere. This report reviews the scientific literature relating to observable, individual-level behavioral indicators that might -- along with other information -- help detect potential violent attacks. The report focuses on new or nontraditional technologies and methods, most of which exploit (1) data on communication patterns, (2) "pattern-of-life" data, and/or (3) data relating to body movement and physiological state. To help officials set priorities for special attention and investment, the report proposes an analytic framework for discussion and evaluation; it also urges investment in cost-effectiveness analysis and more vigorous, routine, and sustained efforts to measure real-world effectiveness of methods. One cross-cutting conclusion is that methods for behavioral observation are typically not reliable enough to stand alone; success in detection will depend on information fusion across types of behaviors and time. How to accomplish such fusion is understudied. Finally, because many aspects of using behavioral observations are highly controversial, both scientifically and because of privacy and civil-liberties concerns, the report sharpens the underlying perspectives and suggests ways to resolve some of the controversy while significantly mitigating problems that definitely exist.