Behavioral threat assessment is a step-by-step process for evaluating individuals who threaten harm to others, making systematic judgments about the seriousness of the threat based on case-specific and dynamic risk factors, and identifying interventions necessary to support the individuals and reduce risk for harming others. On September 3 and 4, 2024, the National Academies of Sciences, Engineering, and Medicine held a workshop on K-12 Behavioral Threat Assessment Efficacy and Implementation Evaluation Research sponsored by the U.S. Secret Service's National Threat Assessment Center. The workshop was designed to explore existing literature on behavioral threat assessment, examine best practices and challenges to implementation, and identify future research needs. This publication summarizes the presentations and discussion of the workshop.