The chapter focuses on the import of the notion of harmony in Middle Platonist theology and cosmology. More specifically, I show that Middle Platonists developed different theological models in order to build up philosophically consistent cosmologies, and one of the main tools applied to achieve these models is the use of the two notions of cosmic harmony and divine harmonisation. On the one hand, a strictly artisanal notion is crucial for those authors who regard God as a divine craftsman and uphold a temporal cosmogony: in their view, God is directly engaged in the production of harmony between opposite cosmological powers. On the other hand, a mathematical and static notion of cosmic harmony paves the way to a sempiternalistic cosmology, denying any direct artisanal intervention in the world on God’s part.