 Medieval philosophers study a variety of wholes and parts, and they often do so with a remarkable degree of sophistication. To be sure, some aspects of medieval mereology are idiosyncratic, but many of the puzzles that medieval philosophers wrestle with are recognizably perennial. Medieval philosophers are particularly attuned to the relationship between mereology and various problems in metaphysics, and many of their solutions to puzzles of identity and survival are embraced by philosophers in other periods. Even the idiosyncratic aspects of medieval mereology reveal a sophisticated appreciation of three fundamental questions in mereology, namely, what items are wholes, what items are parts, and under what conditions is one item a part of another item. This survey can only hint at the richness of medieval mereology. In part, this is due to the overwhelming number of medieval authors who say something or other, somewhere or other, about parts and wholes. But it is also due to the fact that there are still more texts to be unearthed, properly edited, and studied with care. Based on what we have already discovered, we should be confident that we will find many more interesting reflections on parts and wholes and that we will uncover further connections between medieval mereology and more recent theories of parts and wholes.