This is the first comprehensive study of the associations of athletes and artists in the Roman empire. The xystic synod of athletes and the thymelic synod of artists were the only ancient associations that operated on a pan-Mediterranean scale. They were active from southern Gaul to Syria and Egypt and were therefore styled 'ecumenical synods'. They played a key role in Greek festival culture during the imperial period: not only did they defend the professional interests of their members, they also contributed to the organisation of competitions and the maintenance of the festival network. Due to their cultural activities, their connections with the imperial court and their ramified social networks, they left a distinctive stamp on Greco-Roman elite culture during the Principate. Drawing on all available documentation, this book offers new insights into the history and workings of these remarkable associations.