Terrorism, especially lone-actor terrorism, is considered a major national security threat in both North America and Europe. The threat of terrorism has many faces and violence can arise from all ideological extremes. The authors present the theoretical model and current empirical validation of the Terrorist Radicalization Assessment Protocol (TRAP-18), a structured professional judgment instrument for those engaged in risk assessment of persons of concern for acts of terrorist violence. It can be used independently of a particular ideology. The TRAP-18 consists of 8 proximal warning behaviors and 10 distal characteristics, and has been designed to help prioritize the imminency of risk in specific cases, and therefore determine the intensity of monitoring and active management a case requires. Research has demonstrated excellent interrater reliability, and promising content, criterion, discriminant, and predictive validity. More research is in progress. The TRAP-18 is currently used by counterterrorism experts in North America and Europe. It offers a useful approach for professionals who may be assessing and treating individuals of national security concern.