Spinoza claims that God is extended and corporeal, but he resists identifying God with the extended, corporeal world. How then are we to understand the relation of God to the physical world? This essay first critically examines interpretations offered by Schmaltz and Woolhouse which claim that Spinoza's God is not actually extended, but a nonextended essence of extension. It is then suggested that Spinoza's God can be understood as something akin to (a modified version of) scholastic prime matter. On this view, Spinoza's God is actually extended, but cannot be identified with the corporeal world, which is changeable and variegated in a way that prime matter is not.