This is a comprehensive analysis of the idea of libertas and its conflicting uses in the political struggles of the late Roman Republic. By reconstructing Roman political thinking about liberty against the background of Classical and Hellenistic thought, it excavates two distinct intellectual traditions on the means allowing for the preservation and the loss of libertas. Considering the interplay of these traditions in the political debates of the first century BC, Dr Arena argues that, as a result of its uses in rhetorical debates, libertas underwent a form of conceptual change at the end of the Republic and came to legitimise a new course of politics, which led progressively to the transformation of the whole political system.