Polybius (born ca. 208 BCE) of Megalopolis in the Peloponnese (Morea), served the Achaean League in arms and diplomacy for many years, favouring alliance with Rome. From 168 to 151 he was hostage in Rome where he became a friend of Aemilius Paulus and his two sons, and especially adopted Scipio Aemilianus whose campaigns he attended later. In late life he was trusted mediator between Greece and the Romans whom he admired; helped in the discussions which preceded the final war with Carthage; and, after 146, was entrusted by the Romans with details of administration in Greece. He died at the age of 82 after a fall from his horse.

The main part of Polybius's history covers the years 264–146 BCE. It describes the rise of Rome to the destruction of Carthage and the domination of Greece by Rome. It is a great work, accurate, thoughtful, largely impartial, based on research, full of insight into customs, institutions, geography, causes of events and character of people; it is a vital achievement of first rate importance, despite the incomplete state in which all but the first five of the forty books have reached us. Polybius's overall theme is how and why the Romans spread their power as they did.

The Loeb Classical Library edition of Polybius is in six volumes.