This chapter examines Aquinas’s views on the ontological status of the actualization of an agent’s active power, namely its action, and the patient’s passive power, namely its passion. Aquinas claims that one and the same motion constitutes both an agent’s action and its patient’s passion. This chapter considers Aquinas’s motivations for defending the “action-passion sameness” thesis and his responses to common objections. The chapter also includes a solution to a longstanding interpretive difficulty regarding Aquinas’s views on the ontological status of action. Aquinas claims in some texts that actions are accidents in the agent as subject. This seems to conflict with his standard view that an agent’s action is the motion which it causes in its patient. While advancing a solution to this textual difficulty, the chapter proposes a novel interpretation of Aquinas’s understanding of the relationship between forms and accidents and the metaphysics of inherence.