The central relationships in Math uab Mathonwy, the ‘Fourth Branch’ of the Mabinogi, are those between uncles and nephews. Although other connections are also present in the text –between siblings, between mother and son, and between husband and wife – these are presented as subordinate to the more prominent uncle-nephew relationships. The narrative follows the members of a single family, presenting three generations of a very narrow kin-group. Within this family we find multiple uncle-nephew bonds, all of which are between a maternal uncle and nephew, that is, between a man and his sister’s son, or his mother’s brother. Of particular importance are the relationship between Math and the two sons of his sister Dôn, Gwydion and Gilfaethwy, and the relationship between Gwydion and the son of his sister Aranrhod, Lleu Llaw Gyffes.The ambiguous and potentially destructive nature of the avunculate relationship provides a vehicle for the theme of intergenerational conflict and succession, with the emphasis on avunculate rather than father-son pairs as symptomatic of the persistent problems of male fertility and the rule of a sterile king.