Introduction The governments of many countries have used repression against their own and external populations. This has included terrorism. Yet there has been relatively little research on state terrorism within the discipline of international relations and even less on state terrorism by liberal democratic states from the North (Blakeley 2008; 2009). Some scholars even argue that political violence by states should not be classified as ‘terrorism’. I begin by exploring the core characteristics common to existing definitions of terrorism. I show that states should not be precluded as potential perpetrators of terrorism because those core characteristics are concerned with the actions involved in terrorism, rather than the nature of the perpetrators. I then set out the key elements that must be present for an act to constitute state terrorism. I show that a defining feature of state terrorism, and that which distinguishes it from other forms of state repression, is its instrumentality because it involves the illegal targeting of persons that the state has a duty to protect in order to instil fear in a target audience beyond the direct victim(s). In exploring state terrorism in relation to other forms of repression, I show that state terrorism always violates international law because of the methods used to instil terror. Last, I outline the main challenges involved in identifying state terrorism. These relate primarily to questions of agency and motive. Measures that can be taken to overcome these challenges are then proposed.