Aquinas refers to that in virtue of which the patient is acted upon as passive potentiality; and he claims that to every type of active power, there corresponds a determinate type of passive power. This chapter considers Aquinas’s views on passive powers. The chapter first considers Aquinas’s views on the constituents of material substances that give rise to their passive potentialities for being acted upon. Aquinas holds that material substances have passive potentialities in virtue of both their matter and their qualitative forms. The chapter next considers Aquinas’s views on how a material substance’s passive potentialities are identified and distinguished from one another. Finally, the chapter argues that Aquinas thinks that a substance’s passive potentialities for undergoing action are the same as its potentialities for existing in determinate ways. For example, a pot of water’s potentiality for being heated is the same as its potentiality for being hot.