Religious beliefs and practices of the Sumerians and Akkadians, and later of their successors, the Babylonians and Assyrians, who inhabited ancient Mesopotamia .
The deities of Sumer were usually associated with aspects of nature, such as fertility of the fields and livestock. The gods of Assyria and Babylonia , rather than displacing those of Sumer and Akkad , were gradually assimilated into the older system. Among the most important of the many Mesopotamian gods were Anu , the god of heaven; Enki, the god of water; and Enlil, the earth god. Deities were often associated with particular cities. Astral deities such as Shamash and Sin were also worshiped. The Mesopotamians were skilled astrologers who studied the movements of the heavenly bodies. Priests also determined the will of the gods through the observation of omens, especially by reading the entrails of sacrificed animals. The king functioned as the chief priest, presiding at the new-year festival held in spring, when the kingship was renewed and the triumph of the deity over the powers of chaos was celebrated.