"As a statesman a genius of the first order" was Theodor Mommsen's verdict in 1886 on Diocletian, the Dalmatian whose career took him from a released slave to Emperor. Diocletian stabilised the Imperium after it had been thrown into turmoil in the imperial crisis of the period of military anarchy. After his abdication in 305, he retired to the magnificent palace of Spalato (Split, Croatia) built for his old age. Although his arrangements for the succession, his price controls and his anti-Christian policies were not a lasting success, his comprehensive reforms created the basis for Constantine and the transition to the Late Classical Age. Renowned scholars from Germany, Great Britain, Croatia, Slovenia and Switzerland contributed to an international conference held in Split in 2003. Their papers collected here show the present state of research on the Tetrarchy in its political, social, economic, ideological, historico-religious and archaeological aspects and on the reception of Diocletian up to modern times.