 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. (There is very little contemporary scholarship on medieval Arabic, Hebrew, or Byzantine mereology. However, many of the Latin notions and distinctions surveyed in this article surely have correlates in these traditions.) The article will highlight many of the key medieval mereological concepts and principles, and it will outline some of the fundamental issues that confront mereologists in the Middle Ages. Specific philosophers and their doctrines will be used to illustrate some of these concepts, principles, and puzzles. Many of these concepts and principles may seem strange to the modern student of parts and wholes, but behind this alien veneer one will see that medieval mereologists share many of our concerns about wholes, their parts, and the metaphysical implications of mereology.