This chapter discusses Foucault’s use of the texts of Saint Augustine in his work on the history of sexuality in the 1980s, and in particular the importance to that work of the theme of marriage in Augustinian theory. For Foucault, Augustine operates a “libidinization” of sexuality: a concentration of ethical attention, no longer on sexual acts and pleasures, but on desire and the troubled movements of the voluntary and involuntary will. The chapter analyzes this “libidinal structure” through three lines of thought, the development of which can be followed in Augustine’s writings on morality and marriage: the complex role that the dimensions of body and sex play in Augustinian matrimonial doctrine; the elaboration of the concept of flesh (caro), as a focus of problematization and a mode of subjectivation; and the way in which the libido (or “concupiscence”) connects the two poles of the flesh and the body, becoming the fulcrum of the ethical construction of self in Christian culture.