This dissertation offers a new analysis of Polybius of Megalopolis’ Histories by looking at the Achaean historian’s many depictions of treachery and deception during the third and second centuries BCE. Chapter 1 introduces the topic and explores how Polybius’ moral evaluations of treachery consistently center on the personal gain of the betrayer and the context of the situation. Chapter 2 surveys instances of treachery and deception taken against friends and allies in the Histories, which are remarkably even-handed. Polybius makes allowances for certain betrayals in the face of extreme necessity and wicked allies, and he is especially critical of treachery committed despite past kindnesses. Chapter 3 discusses Polybius’ accounts of treachery and deception carried out against one’s own state. In addition to showcasing continued Polybian contextual nuances, the chapter emphasizes the frequency and severity at which these internal betrayals occur in monarchies, when compared with republics, providing insight into Polybius’ attitude toward monarchies, republics, and the morally corrupting weight of power on different forms of government. Significantly, the Romans remain unique in the Histories in their utter lack of any such internal treachery. Chapter 4 explores instances of diplomatic deception employed during times of war, maintaining that Polybius allowed for a significant amount of diplomatic deception and maneuvering when a formal treaty or truce had not been sworn. In the Histories, when oaths are sworn, the Romans are typically the victims of such treachery, often characterized by Polybius as παρανομία. The exception to this is Polybius’ condemnation of Marcius Philippus’ deceptions against Perseus in 172/171, which seems inconsistent given Polybius’ heavy praise of Scipio in 203 and Flamininus in 198/197 for strikingly similar deceptions. This deviation matches Polybius’ efforts throughout the Histories to fabricate a contrast of earlier Roman virtue with contemporary Roman depravity. Chapter 5 adopts a “Greek vs barbarian” lens to argue that Polybius connected the acquisition of power with the moral decline and political collapse of the state – both of which are forecasted by Polybian descriptions of παρανομία. Notably, throughout the Histories the Romans never exemplify such παρανομία. The interconnection between moral corruption and pragmatic consequence is emphasized in Polybius’ descriptions of the many unjustified instances of treachery throughout his work, all of which are carefully shown to have dire consequences for the culprits. The dissertation concludes by arguing that Polybius realized that in the new unipolar reality of Roman domination, the only check on tyrannical Roman behavior would have to come from the Romans themselves. Thus, Polybius uses his fabricated model of past Roman virtue and present Roman depravity to challenge the Romans to surpass the benevolence of their ancestors, while warning the Romans about what would happen should they resort to “barbaric” tyranny. These didactic efforts suggest that Polybius wrote the Histories with a Roman readership in mind.