This essay explores the changing definitions and perceptions of “hermaphrodites” from the colonial period to the early twentieth century. Over the course of the three centuries, most medical observers would have agreed that hermaphrodites did not exist in the human species and that patients with confused or ambiguous external and internal reproductive organs were not really hermaphrodites, but cases of “mistaken sex.” Indeed, by the mid-twentieth century, “corrective” surgery for such anatomical ambiguity became routine in this country, to make infants’ genitalia look “normal” and match their supposed “true sex.” But this essay is concerned less with the medical history of surgical procedures or the professional history of doctors than with the cultural history of how American doctors and laypeople regarded bodies and identities that fell outside their conceptual boundaries of normal female and male categories. What did it mean to be male or female? Who had the authority to answer that question, and what were the criteria? The essay is organized chronologically as well as thematically, for the classification of hermaphrodites, impossible though the status was thought to be, changed over time. Each section explores the evolving determination of the biological and social foundations of sexual identity and the anxiety, expressed differently in different eras, over those cases that did not fit the ideal bipolarity.