The medieval jurist Bartolus of Sassoferrato (d. 1357) has long been accorded seminal importance by historians of political thought. This volume provides the first complete English translation of his three most celebrated tracts: On Guelfs and Ghibellines, On the Government of a City, and On the Tyrant, which constituted the first consolidated response by a medieval lawyer to the problem of tyranny in the city republics of central and northern Italy. Crucial sections of Bartolus' academic commentaries on Roman law are also translated in an appendix. George Garnett and Magnus Ryan make the writings of Bartolus accessible to an expanded audience, situating his political theory in its original context and explaining his arguments. Footnotes to the translation explain all Bartolus' references to normative sources, legal and otherwise, and a detailed glossary of legal terms and institutions is provided. This translation allows readers to understand how Bartolus mobilised the Roman and canon laws to address immediate political developments, and why he was the most famous and enduringly influential medieval lawyer.