This chapter provides an introduction to Aquinas’s views on efficient causation and causal powers, as well as some background and context necessary for appreciating his views. The chapter first introduces Aquinas’s views on the nature of the relationship between an efficient cause and its effect and the various elements involved in paradigm cases of efficient causation. After presenting an overview of Aquinas’s theories, the chapter next contrasts Aquinas’s views with competing historical theories of causation. Comparison with these other theories helps to highlight what is philosophically significant in Aquinas’s theories. The chapter also discusses Aquinas’s sources and situates his views relative to medieval debates about causation. This background provides some context for appreciating what is original or controversial in Aquinas’s theories. Finally, the chapter includes an introduction to the technical terminology that Aquinas uses to express his views on efficient causation and causal powers.