Treason has been ubiquitous in human history, stretching from the crime of maiestas in the Roman empire to the ‘whistle-blowing’ behaviour of Edward Snowden. It usually manifests itself in the public domain either as the worst crime imaginable, or as the bitterest example of political rhetoric. Yet conceptual studies of treason are surprisingly few compared to those about loyalty. My research (my book project) analyses the evolution of treason in one particular context, the late Habsburg monarchy (1848-1918), and aims to position that era in a broader history of treason. For a regime notoriously beset by domestic and foreign threats, we can ask, when and why was ‘treason’ identified? What criteria were used in prosecuting the perceived traitors? And what were the implications for the long-term relationship between the state and its citizens? I will be focusing mainly on treason in its legal framework as a political crime, using examples from the 1848 revolutions through to the end of the Monarchy. The 1848 revolutions and their neo-absolutist aftermath illustrate the complexity of defining and prosecuting treason. In the twentieth century, the Zagreb treason trial of 1909 was compared at the time to France’s infamous Dreyfus Affair, yet has never been properly researched by historians. Both these cases reveal much about state insecurity. Both also show the potential dangers for long-term stability when the authorities resorted to treason law in order to monitor and punish ‘disloyalty’. For as the Habsburg regime discovered by 1918, once traitors were identified and prosecuted it was very dangerous to suggest they could be amnestied and allowed unconditionally back into society.