Dio Cassius of Nicaea was a Roman senator and historian of Greek origin from Nicaea in Bithynia. He lived from approximately 164 to 229 CE. He pursued a successful political career, achieving the consulship twice. His career spanned the reigns of Commodus through Severus Alexander, during which he also served as a provincial governor. He dedicated over two decades to researching, writing, and revising his monumental history before retiring to his native Nicaea.

His principal work is the 80-book Roman History, covering Rome from its mythical origins to 229 CE. Substantial portions survive for Books 36 through 60, while the rest is known through later summaries and epitomes by John Xiphilinus and John Zonaras. He also authored an earlier, shorter work on the dreams and portents that foretold the rise of Emperor Septimius Severus.

Dio Cassius provides a crucial insider’s perspective on imperial politics and senatorial procedure. His analytical history, written in Atticizing Greek, explains events through constitutional change and the decline of republican institutions. His work forms a vital historical source for the period of the late Republic and early Empire.