Over 500 Scots journeyed to Spain to fight on behalf of the Spanish Republic as part of the International Brigades, alongside some 35,000 other foreign volunteers. Their decision to personally intervene in the Spanish Civil War placed these Scots at a crucial juncture in history. They formed part of the single largest mobilisation of transnational foreign fighters in modern history, represented the apex of interwar anti-fascist activism and posed a complex security dilemma for the British state on their return. In examining the Scottish volunteers' decisions and their consequences, this thesis contends that existing historical explanations of the International Brigades' recruitment and organisation have significant limitations. Crucially, previous accounts have failed to appreciate the extent and importance of pre-existing social and political networks among the volunteers, which were facilitated by Scotland's particular political cultures in the interwar period. Moreover, examination of the Scots' time in Spain sheds new light on the International Brigades themselves, including their political organisation, the handling of dissent, desertion and disaffection and the volunteers' relationships with Spanish civilians and conscripts. Finally, the post-civil war trajectories of the Scottish volunteers indicate limitations to enduring popular and historical narratives of their victimisation as 'premature anti-fascists' at the hands of the British state.