This article gives an overview of the mereological thought of the medieval philosopher and theologian Saint Thomas Aquinas. Until the 20th century mereology—the study of the relationship of whole and part—was not generally recognized as a distinct domain of philosophy and so Aquinas, like most of his contemporaries, has no single systematic treatment of it as a topic in its own right. However, a cohesive account of part‐whole relations can be pieced together from the various metaphysical, ethical and theological contexts (...) in which Aquinas deals with mereological issues. In this article, I give an account of both Aquinas's conceptual understanding of composition and his taxonomy of the different kinds of wholes and parts that there are. I end with comments on three marginal cases of mereological relationships in Aquinas's philosophy. (shrink)