Constant Mews considers what we know about the personal libraries collected by individuals during the twelfth century. Because most medieval manuscript books survive through having been bequeathed to institutional libraries, whether of a cathedral or monastery, the scale of individual private libraries in the twelfth century tends to be hidden from view. From surviving book lists and records of bequests, we learn that there was a dramatic growth in this period in the collection of literary and scientific texts, as well as in patristic and scholastic theology and exegesis.