This dissertation focuses on changes in the form and function of kinship as a means of analyzing larger social and cultural changes in the tenth through twelfth centuries in Byzantium. The tenth and eleventh centuries witnessed the rise to prominence of a group of aristocratic families in Byzantium who originated outside of Constantinople and whose wealth and influence were not entirely dependent upon the support of the imperial state. These families brought with them certain social and cultural norms that differed significantly from earlier political elites, in particular their deep-seated sense of loyalty to and identification with their extended family group or lineage, which surviving Byzantine histories, wills, court records, and lead seals refer to as their "genos." The influence of these families and their ideas reached the very pinnacle of Byzantine socio-political organization with the ascent to the throne of Alexios I Komnenos in 1081, whose dynasty's reorganization of the imperial bureaucracy over the following century saw the bonds of kinship take precedence over older systems of political order. For decades, modern scholars have interpreted these changes as the 'expansion' of the Byzantine family, though the precise details of this phenomenon remain obscure. The central goal of my research is to ascertain the role and function of the Byzantine aristocratic family group, or genos, as a distinct social entity, particularly its political and cultural role, as it appears in a variety of sources in the tenth through twelfth centuries. The genos was arguably the single most fundamental element of both individual and collective identity in Byzantium from at least the tenth century, if not earlier. It was the dominant form of the family as a singular social unit in questions of inheritance and marriage, and it formed the basic building block of political alliances within the aristocracy of Byzantium in this same period. The bond formed by shared membership in a single genos was among the strongest of any social bond in Byzantium. It was both affected by and instrumental in the social and cultural changes occurring in Byzantium in the tenth and eleventh centuries. In these ways, the genos was at least as, if not more important than the oikos/household, yet it remains much less well understood among modern historians.