This dissertation examines the role of secret practices of opposition in the urban politics of Florence between 1340 and 1382. It is based on a wide variety of printed and archival sources, including chronicles, judicial records, government enactments, the records of consultative assemblies, statutes, chancery letters, tax records, private diaries and account books, and the ad hoc opinions (consilia) of jurists. Over the course of four chapters, it presents three major arguments: (1) Conspiracy, a central mechanism of political change and the predominant expression of political dissent in the city, remained primarily a function of the factionalism that had shattered the medieval commune, although it was now practiced not as open warfare but secret resistance. (2) Conspiracies were especially common when the city was ruled by popular governments, which faced almost constant conspiratorial resistance from elite factions that been expelled from the city or had had their political power restricted, while also inspiring increased worker unrest and secret labor organization. (3) Although historians have often located the origins of the “state” in the late medieval and early Renaissance cities of northern and central Italy, the prevalence of secret political opposition, the strength of conspirators and their allies, and the ability of conspiratorial networks, large worker congregations, and even powerful families to vie with weak regimes for power and legitimacy seriously calls this into question.