Una caro “One flesh” is the way the Bible expresses marital unity. In ordinary language, however, “the two of us” is an expression that appears when there is a strong sense of reciprocity within a marriage—that is, an awareness of the beauty of an exclusive love, of an alliance between two people who share life in its entirety, with all its struggles and hopes. A person says “the two of us” when referring to desires, sufferings, ideas, and dreams held in common—in a word, to the stories that only spouses have lived. This is a verbal manifestation of something deeper: a conviction and a decision to belong mutually to one another, to be “one flesh,” to walk the path of life together. As Pope Francis has said: “Spouses too should form a first-person plural, a ‘we’. To stand before each other as an ‘I’ and a ‘you,’ and to stand before the rest of the world, including their children, as a ‘we’”. This happens because, although they are two different persons, two individuals each possessing their own unrepeatable identity, they have forged by their free consent a union that places them together before the world. It is a union that opens generously to others, but always from that unique and exclusive reality of the conjugal “we.” Saint John Paul II, speaking about monogamy, stated that it “deserves to be ever more deeply explored”. His indication of the need for a more extensive treatment of this theme is one of the motivations that led the Dicastery for the Doctrine of the Faith to prepare this doctrinal Note. Also at the origin of this text are, on the one hand, various dialogues with the bishops of Africa and other continents on the question of polygamy in the context of their ad limina visits, and, on the other hand, the observation that various public forms of non-monogamous unions—sometimes called “polyamory”—are growing in the West, in addition to the more discreet or hidden forms that have been common throughout history.