In this chapter, we consider environmental, cultural and cognitive constraints on the transmission of religious representations in the ancient Greek world. Although Greek religion is often described as “conservative,” it was characterized by constant change, even as its intuitive foundations, including anthropomorphism and reciprocity, remained stable over centuries. The Greeks viewed themselves as faithful custodians of unchanging ancestral traditions, but many forms of change were invisible to them while others were welcomed because of the nature of polytheism as an open system. The chapter includes a brief outline of key developments from the Early Iron Age through the Hellenistic period. We conclude with an examination of the processes by which representations spread across cultures. “Syncretism” and “Hellenization” are contested terms, yet widely used by scholars describing various forms of cultural blending. The illustrative essays examine continuity in the cult association of the Molpoi at Miletos, the spread of cults of Herakles through the Mediterranean world, and the reasons for the growth of the Sarapis cult in early Ptolemaic Egypt.