This is a study of Julio-Claudian Rome’s relations with Parthia, one of the great empires of ancient Iran and an imperial rival of the Principate. In contrast to previous scholarship that treats Rome and Parthia as monoliths clashing in war or coexisting through diplomacy, “The Arsacids of Rome” shows how their relationship changed through interconnection at the highest levels of imperial power. Its approach centers on a group of people whose careers spanned both empires: the Parthian hostages who lived in the city of Rome. These men and women were members of the Arsacid family, Parthia’s ruling dynasty. Shipped abroad by the reigning Arsacid monarch, they lived for extended periods in the city of Rome before returning to their native land. Through their influence, it is argued, internal and external affairs in both empires were linked together. Hostages enmeshed and entangled the interests of kings, emperors, aristocrats, and governors on both sides of the Euphrates, creating coalitions and constellations of power across the political boundary that divided Parthian territory from Rome. Their careers show how Rome and Parthia confronted one another not as two solitudes, but as two porous and permeable entities that were interconnected even in the realm of high politics.