There is no empirical evidence to suggest that terrorism is driven by mental illness in the majority of cases. However, when terrorist acts are planned or executed by individuals with mental disorder, possible functional links between the two need to be explored in order to delineate risk and inform approaches to risk management and reduction. This paper explores such functional links, their complexities and implications for clinical interventions, with a focus on psychosis, PTSD and addictions. The challenges of establishing the precise role of mental disorder, especially where there is co-morbidity and a range of complex interacting symptoms, are explored. Finally, the limitations of the existing research in the field of mental disorder and terrorism, and the challenges of extrapolating from such research to practice, are addressed.