In the eleventh and twelfth centuries, medicine in the Mediterranean was characterized by intellectual ferment and rapid change, characteristics quickened by substantial cross-cultural influences. In the Latin West, for example, translations produced in the long twelfth century (from the late eleventh to the early thirteenth centuries) marked a major shift in medicine's development into a learned profession and also reshaped understandings of the human body. This dissertation explores both the cross-cultural circulation of medicine in the medieval Mediterranean and its place in Byzantium and the Latin West. First, it examines medicine in eleventh- and twelfth-century Byzantium, both because of its considerable vitality and because Byzantium's broad influence and prestige make it an essential context for the Latin West. Rather than characterized by newly ascendant medical practitioners, as has been suggested, a divide appears to have existed between the enthusiasm for medicine among learned readers and the humbler, often empirical world of Byzantine medical practitioners. Second, the translations of Constantine the African are examined, including both the contexts of their production and their connections to the other worlds of Mediterranean medicine. In particular, a detailed examination of Constantine's Viaticum is undertaken, a translation of the Zād al-Musāfir of Ibn al-Jazzār, with particular attention to its translation methods and terminology. The Viaticum reveals the complexity of Constantine's circumstances, where Constantine balanced his personal inclinations against the sensibilities and ideological aims of the monastery at Monte Cassino that financed his efforts. Further, the Viaticum's terminology is examined and systematically compared with both ancient and contemporary Arabic, Latin, and Greek sources and analogues. In contrast to earlier interpretations, the Latin character of this terminology appears particularly pronounced; this suggests influence from contemporary Salernitan medicine is present, but perhaps also some distance. Finally, the reception of Constantine's works is explored in William of St.-Thierry. Although often seen as highly conservative, William's work on the nature of the body reveals an effort to fuse medical and theological accounts of cognition, with suprising results. In sum, this dissertation reveals much about developments in Mediterranean medicine in the eleventh and twelfth centuries and its broad cultural impact.