This is the first volume of the three-volume Prosopography which provides a complete secular biographical dictionary for the Later Roman Empire from AD 260 to 641. It begins with the capture of Valerian by the Persians in 260 and ends at the death of Theodosius I in 395. Like its predecessors, this volume has collected the surviving evidence about the personnel of the empire, about members of the senates of Rome and Constantinople and their families, and about members of senatorial families still surviving and holding public office in the western lands (Gaul and Spain) no longer under Roman rule. The work also includes non-Romans who entered imperial service or who in other ways figure in imperial history, among them many foreign rulers. The project is intended as a tool for research workers in the whole field of late Empire studies, and will be an indispensible work of reference for years to come.