This chapter is about Aquinas’s views on active powers. Aquinas uses the Latin term potentiae to refer to active powers in general. Accordingly, the chapter begins with Aquinas’s understanding of the distinction between potentiality and actuality and the different types of potentialities. The chapter next considers his views on how active potentialities are individuated. Aquinas claims that active potentialities are distinguished by the acts that immediately arise from them. The chapter then examines Aquinas’s views on what active powers are ontologically. Aquinas identifies active power with form. Forms are both that by which a substance is actual in a determinate way and that through which a substance is capable of causing the same type of form in another substance. Although Aquinas thinks that active powers are forms, he denies that every form is an active power for material change. For example, the form of redness is not an active power for making other substances red. The final sections of the chapter discuss Aquinas’s views about which forms are and are not active powers for initiating material change.