This dissertation explores the Roman idea of pietas in the late 1st century BC. It concentrates on the development of the idea from a marker of familial and religious responsibilities to its usage as a political tool of the late Roman Republic. The work examines the philosophical writings and letters of Cicero, the histories of Appian and Cassius Dio, and the Latin poetry of the late Republic and early principate. I will argue that pietas functions on a basis of gratitude which prompts obligation and reciprocal duty. These characteristics will be shown to be the basis of Roman familial and political relationships. Cicero’s philosophical writings on the term will be compared to his personal letters to show the distinction between the ideal of pietas and its actual, pragmatic usage at the end of the Roman Republic. In turn, pietas will then be shown to be a tool used by those who had a hand in the reordering of the Roman state. Those who sought to advance their own claims to power utilized pietas as an articulation of the mutual debts shared between leaders and subordinates. This was done within the framework of traditional Roman views of the obligations and duties inherent in their relationships. This language of mutual responsibility characterizes the Roman understanding of pietas. This ancillary language of debt and reciprocity, obligation and responsibility signifies expressions of pietas. Both the Latin and Greek terms associated with pietas show that it maintained a fixed set of principles but nevertheless came to be used to a variety of ends. Finally, I will turn to the poets of the end of the 1st century BC to show the use of pietas as an expression of a complicated set of principles important to the Augustans. Pietas was used to define a justification for the new principate after the civil wars. This principate was represented by these poets as underpinned by a traditional relationship among the gods, family and the state. These relationships, despite their new political context, were still governed by traditional Roman religious and familial notions of mutual obligation and pietas.