The ethics of Thomas Aquinas should be counted among the most fruitful and influential approaches to moral philosophy. It is often seen as the medieval counterpart to the towering achievements of ancient and modern ethics produced by thinkers like Aristotle and Immanuel Kant. This chapter provides a rough sketch of Aristotle's influence on Aquinas's ethics. It provides views on Aquinas's commentary on the Nicomachean Ethics (EN), the Sententia libri Ethicorum. Aquinas's views on the will imply that the systematic structure of his action theory and ethics differs from Aristotle's in an important respect. Aquinas does not merely assume that the will, as a rational appetite, is a distinct power of the soul. For Aquinas, the most fundamental action of a human being is the internal act of the will, which is completed when the will accepts one course of action proposed by reason (Perkams).