It is now a cliché to observe that, despite innumerable efforts to define terrorism, scholars are no nearer to arriving at a consensus. One approach has been to assemble the multitude of definitions produced by academics, commentators, governments and international organisations, in an attempt to identify common ground. There is much to admire in this ethos of catholicity – though the final product can feel more like a catalogue of component parts than a cohesive and workable definition. How, then, should historians approach the debate over how to define terrorism? To explore this question, this chapter begins by reflecting on the ‘genealogical turn’ in historical method – a development closely associated with the ‘Cambridge school’ of intellectual history. It will then attempt to construct a genealogy for ‘terrorism’, as reflected in existing historical narratives of this subject. And finally, it will consider what key themes emerge from such a genealogical examination.