[Revised entry by Andrew Arlig on August 9, 2023. Changes to: Main text, Bibliography, notes.html] The term "mereology" is sometimes used to refer to any one of the formal languages that describe the part-to-whole relation. In this article, I will use the term more broadly to refer to any theoretical study (formal or informal) of parts, wholes, and the relations (logical or metaphysical) that obtain between them. In what follows I will survey some of the ways that philosophers in the medieval Latin West thought about parts and wholes. (While there is very little contemporary scholarship on medieval Arabic, Hebrew, or...