The online database published here is the first major result of Prosopography of the Byzantine World (PBW), a project covering the period AD 1025-1180, and represents a continuation of prosopographical work originally inspired by A.H.M. Jones in 1950, and sponsored since then by the British Academy. The Prosopography of the Byzantine Empire aims to record all surviving information about every individual mentioned in Byzantine textual sources, together with as many as possible of the individuals recorded in seal sources, in the period 642-1261: This general statement covers more than 600 years, a variety of periods with different characteristics, and it has become necessary to qualify it, in three ways: in connection with seals sources, with historical changes in Byzantium, and with the cultures and languages covered. The core of the database is made up of nearly 60,000 factoids (small pieces of information classified under different categories), each of which is linked to an owner and (generally) at least one other secondary person by a hypertext link. More than a third of the factoids are of the narrative type, and these are organised into narrative units by further links. There are 2,774 such units. The units are in turn linked to dates and reigns, and some of them to larger events and problems. There are, in addition, around 7,500 seals, with links to matrices which number 5,000. Each seal is linked to a museum or private collection and at least one edition, and each set of matrices to an owner, certain or hypothetical, in the core of the database.