This chapter raises the question of the nature and roots of Babylonian horoscopy and genethlialogy. It considers the fundamental difference between Babylonian and Greek/Greco-Roman astrological practices, devoting particular attention to the ways the world was thought to work in terms of a causal structure underpinning the idea that human life was indicated or even determined by the heavens. Recognition of this essential metaphysical distinction in causality is, I suggest, essential to our historiography of early astral sciences as well.