Powers explain how agents are able to act. Yet, Aquinas thinks that we must posit something further in a natural agent to account for why it exercises its power whenever it is possible to do so. Natural inclination is that impetus within natural agents that determines them toward action. This chapter examines Aquinas’s understanding of natural inclination and the role it plays in efficient causation. The chapter first considers Aquinas’s views on what natural inclinations are and why they are necessary. The chapter next considers his views on how natural inclinations explain how natural agents act for the sake of ends. Even though natural agents cannot know the ends for which they act, Aquinas thinks that they nevertheless act for the sake of goals through their natural inclinations. Lastly, the chapter examines Aquinas’s views on the ultimate cause of natural inclinations. Aquinas maintains that natural inclinations, and the natures upon which they follow, must have their ultimate causal source in a being with cognition, namely God. The chapter analyzes Aquinas’s rationale for this view.