This chapter uses text from throughout Aquinas’s corpus to reconstruct the main elements of his views on causation. Causation for Aquinas is a type of ontological dependence. Following Aristotle, Aquinas recognizes four species of causes. The chapter focuses in particular on efficient causation since this is the type of cause that most closely corresponds to what contemporary philosophers mean by a “cause.” Aquinas thinks that efficient causes act through active casual powers to bring about their effects. To highlight the philosophical significance of Aquinas’s views, the chapter compares Aquinas’s views on efficient causation with two prominent contemporary theories of causation, Humeanism and Nomicism.