This chapter investigates the development of the ecumenical synods in the first century ad, first discussing the xystic synod of athletes and then the thymelic synod of artists. After giving an overview of earlier scholarly opinions on their development, this chapter discusses the evidence from a new perspective. It argues, for instance, that there had been only one athletic synod all along, rather than two athletic synods that eventually merged. As for the thymelic synod of artists, it is clear that it was already fully formed and functional in the first century ad, which opposes the interpretation of some scholars who date its definitive formation about a century later. All in all, the sources indicate that the synods gradually strengthened and expanded their role in the festival world throughout the first century ad and that this evolution gained momentum in the reign of Claudius (AD 41–54).