Although the years since 9/11 have seen an significant increase in the contribution of criminologists to the study of terrorist events, efforts to apply major criminological theories to the understanding of the development of terrorist criminality and individual involvement in terrorist action have lagged. In this chapter, we apply a recently formulated theory of moral action and crime causation, Situational Action Theory, to the explanation of terrorism and radicalization. The case is made that explanations of terrorism and radicalisation should be mechanism-based and integrate all levels of analysis. Situational Action Theory is introduced and examples of its application to the study of terrorism and radicalization are provided. The priorities of a SAT-driven, systematic research agenda are outlined.