The emperor Theodosius I (AD 379–395) was one of the most remarkable figures of the late antique period. In the face of religious schism, political turmoil, and barbarian threats he managed to maintain imperial power and forge a political dynasty that would dominate both east and west for over half a century. This study, the first English language biography in over twenty years, traces his rise to power and tumultuous reign, and examines his indelible impact on a rapidly changing empire.