Beliefs and practices of the peoples who inhabited ancient Mesopotamia (now in Iraq) in the millennia before the Christian era, including Sumerians and Akkadians, and their successors, the Babylonians and Assyrians. These religious beliefs and practices form a single stream of tradition that was Sumerian in origin, but augmented and modified by the Akkadians, who were Semites who emigrated into Mesopotamia from the west at the end of the 4th millennium BCE.