An increasing fear over violence in schools has prompted recent requests for psychologists, educators, and law enforcement professionals to assist in preventing future incidents of violence. This paper attempts to lay a foundation for developing effective assessment and prevention approaches by first distinguishing planned school-based attacks from other forms of school and youth violence. The three assessment approaches that have been advocated in some districts (profiling, guided professional judgment, and automated decision-making) are shown to be inappropriate and potentially harmful. The contours of the threat assessment approach, developed by the U.S. Secret Service to prevent assassinations, is examined for its utility in responding to communications or behaviors of concern that students may present in school settings. The most effective approach for understanding and preventing planned school-based attacks will be the one informed by empirically-derived knowledge about the antecedents, motive, idea development, communications, and planning behaviors of all known perpetrators of targeted school violence. (Contains 68 references.) (JDM)