This thesis examines the violent practices which took place in Republican-held territories during the Spanish Civil War (1936-1939), especially during the early months of the conflict. In order to understand the logic and motivations behind violence, as well as its political meaning and its performative dimension, this work combines an overview of the phenomenon in the Republican zone as a whole with a series of detailed studies of specific locations. The thesis deals with the related issues of violence, politics, revolution and justice in the wider context of inter-war Europe. It provides a political interpretation of violent practices, which stresses that the demands for, and the control and management of, violence, were key issues in the definition and construction of the Republican rear guard. Violent practices were largely determined by war-related events and trends; they were also instilled with symbolic and cultural meaning by previous social conflicts and political identities, as reflected in anticlericalism. That said, the intensity, pace and regional differences of violent episodes were heavily determined by an early process of atomisation of political authority and a subsequent reconstruction of the state’s central police and judicial structures. This works points out that most repressive practices were perpetrated by a multiplicity of new political agents who operated outside the margins of the state. The cases of Aragón, Toledo and Vizcaya show a close relationship between the multiplication of political agents in possession of actual authority and the intensity of violent episodes. This relationship was based upon the redefinition of the criteria for political legitimacy and the contentious ideas built around the notion of 'popular justice’. The pockets of authority which emerged at the outbreak of the Civil War were not only a vehicle for private hatred and greed, but also a - bloody - way to gain a prominent position in the new society to come, and to pursue different criteria of social justice.