Theories are systematic and evidenced-based approaches to processes and phenomena. They help us categorize, understand, and predict outcomes. This article argues that multi-theoretical approaches to problem-solving and research trump single-theory research. The article chronicles the multi-theoretical approach to terrorism, crime, and mass shootings in an undergraduate homeland security course. The course applies five theories: social movement theory, social identity theory, radicalization, general strain theory, and lone-actor grievance-fueled violence to terrorism, crime, and mass shootings.