The Introduction gives a brief account of Bartolus’s life, explains the world of medieval law in which he worked, and then explains the political context of the northern and central Italian city republics for which he worked, and whose problems he sought to analyse. It explains that tyranny was Bartolus’s main preoccupation, even in the two treatises ostensibly concerned with other questions. It then presents the main arguments of his three political treatises and Bartolus’s main political theory in his academic legal commentaries, and describes the later influence of these treatises in European political theory. The Introduction also argues that Bartolus conceived of these three treatises as one composite treatment of tyranny.