Either/Or (Danish: Enten – Eller) is the first published work of the Danish philosopher Søren Kierkegaard. Appearing in two volumes in 1843 under the pseudonymous editorship of Victor Eremita (Latin for "victorious hermit"), it outlines a theory of human existence, marked by the distinction between an essentially hedonistic, aesthetic mode of life and the ethical life, which is predicated upon commitment.

Either/Or portrays two life views. Each life view is written and represented by a fictional pseudonymous author, with the prose of the work reflecting and depending on the life view being discussed. For example, the aesthetic life view is written in short essay form, with poetic imagery and allusions, discussing aesthetic topics such as music, seduction, drama, and beauty. The ethical life view is written as two long letters, with a more argumentative and restrained prose, discussing moral responsibility, critical reflection, and marriage. The views of the book are not neatly summarized, but are expressed as lived experiences embodied by the pseudonymous authors. The book's central concern is the primal question asked by Aristotle, "How should we live?" His motto comes from Plutarch, "The deceived is wiser than one not deceived."

The aesthetic is the personal, subjective realm of existence, where an individual lives and extracts pleasure from life only for their own sake. In this realm, one has the possibility of the highest as well as the lowest. The ethical, on the other hand, is the civic realm of existence, where one's value and identity are judged and at times superseded by the objective world. In simple terms, one can choose either to remain oblivious to all that goes on in the world, or to become involved. More specifically, the ethic realm starts with a conscious effort to choose one's life, with a choice to choose. Either way, however, an individual can go too far in these realms and lose sight of their true self. Only faith can rescue the individual from these two opposing realms. Either/Or concludes with a brief sermon hinting at the nature of the religious sphere of existence, which Kierkegaard spent most of his publishing career expounding upon. Ultimately, Kierkegaard's challenge is for the reader to "discover a second face hidden behind the one you see" in themself first, and then in others.