In this doctoral dissertation Father Griffin proposes a theological and anthropological rationale for priestly celibacy as a privileged mode of living supernatural fatherhood and as a fulfillment of masculine human nature. Through the theological lens of St. Thomas Aquinas, it begins by exploring the phenomenon of paternity itself, beginning with the prime analogue of fatherhood in God and cascading into the biological, natural, and supernatural expressions of fatherhood in men. The specific supernatural fatherhood of the priest is derived from his ordination, by which he is configured to Christ the Head and becomes an instrument of Christ's fatherhood in the order of grace, exercising his ministry in the triple office of priest, prophet, and shepherd and fulfilling, within the plan of salvation, both the procreative and perfective dimensions of human paternity. By demonstrating that priestly celibacy is ordered to the exercise of this supernatural fatherhood, celibacy is shown to be not an arbitrary clerical discipline but inscribed with its own natural and supernatural logic, an effective way of generating life in the order of redemption and the normative mode of exercising priestly paternity. Throughout the work, counterarguments, particularly those proffered by feminist theologians, are considered and addressed. The concluding chapter considers some implications of this thesis in the life of the priest and in the selection and formation of candidates to the priesthood.