This article examines ways in which the existence of incestuous relationships at the heart of the utopian project at the end of the Eighteenth century poses a philosophical challenge. When a utopian community is built on the principle of a definite improvement, how is it possible to envision a positive future when incest is introduced at the foundation of a society that pretends to be better than the existing one? Furthermore, how is it possible to establish laws and rules in a society when the initial members of this society have not been able to respect a law supposedly inviolable? Lastly, how can a utopian community, which thrives to be an ideological model, adopt incest as a legitimate possibility or even a norm? This article explores how several prerevolutionary utopias try to answer all those questions. Among the texts it considers are Guillaume Grivel’s L’Isle inconnue, Giacomo Casanova’s L’Icosameron, and Jacques Henri-Bernardin de Saint-Pierre’s Paul et Virginie.