Prominent terrorism case studies of individuals such as Omar Mateen, Dylann Roof, and Mohammed Merah indicate the need for personality trait-based terrorism risk assessment/threat assessment (TR/TA). This chapter provides an overview of Corrado’s, personality-based TR/TA instrument (see Chapter 14) by explaining the origin of each domain and the purpose of inclusion. Furthermore, this chapter displays results from a preliminary instrument validation study conducted on an open-source sample of 158 terrorists. Results of this study suggest strong statistical significance for many of the domains. This suggests the need for future inclusion of personality-based indicators in terrorism risk assessment.