Parallel with Rome becoming a European diplomatic centre, through the fifteenth and sixteenth centuries, European princes and Italian nobles sought to place their sons and the relatives of their advisors in the cardinalate in order to place a permanent representative at the papal court, gain prestige, and access ecclesiastical wealth. In a significant number of cases these men were below the canonical age of thirty, which popularly linked the practice to papal nepotism and prompted criticism of underage clergy as a persistent institutional abuse in need of eradication. This study investigates the geographic and familial origin of underage cardinals elevated to the College between 1420 and 1605. The evidence shows that initially papal kin comprised the majority of underage cardinals, but that after 1470 European royal and Italian noble families far outstripped papal kin in providing underage cardinals to the College. This practice was adopted broadly for its pragmatic value, even though it remained a target of criticism through the age of Tridentine reform.