Polybius of Megalopolis was a Greek statesman and historian of the Hellenistic period. Born around 200 BCE into the political elite of the Achaean League, he was taken to Rome as a hostage in 168 BCE and remained there for seventeen years. During his detention, he formed a close association with the Roman general Scipio Aemilianus. This connection granted him direct access to Roman military and political circles, an advantage he later used to assist in the political settlement of Greece after 146 BCE.

His major work is The Histories, a 40-book universal history chronicling Rome's rise to dominance from 264 to 146 BCE. Only the first five books survive in their entirety, with substantial fragments of others remaining. Polybius also authored several other works, now lost, including a biography of the Achaean statesman Philopoemen, a treatise on military tactics, a history of the Numantine War, and a geographical study on the habitability of the equatorial region.

Polybius is a historian of paramount importance, as his work provides the principal surviving contemporary narrative for Rome's rapid expansion during this critical period. He championed a rigorous, pragmatic historical methodology, emphasizing eyewitness accounts and the analysis of cause and effect. His analysis is uniquely bicultural, offering an outsider's perspective on Roman institutions. His most famous and influential contribution is his theory of the Roman mixed constitution, which he identified as the key source of the republic's political stability and strength.