The focus of this chapter concerns the performance of legitimacy, or rather its opposite, the performance of usurpation and tyranny.1 Medieval and early modern political theory has been recently magisterially synthesized by Francis Oakley. However, the influence of the Great Western Schism on political theory has been largely ignored, outside discussions of concilarism.2 The following will engage the Schism with the issue of legitimacy and eventually tyranny and tyrannicide. After discussing medieval concepts of tyranny, this chapter will examine if contemporary authors considered schismatic popes to be tyrants. Once established that in fact they did, the chapter will investigate further the consequence of the latter, looking at contemporary cases of so-called tyranny, with accusations against Richard II and Louis of Orléans.