The Enlightenment sought rational order in the world and found irrationality at every turn, especially where sex was concerned. The human body and its desires exposed powerful contradictions in an age inclined to systematic thinking. This short book surveys some varieties of sexual inquiry and anxiety that marked European culture during the eighteenth century: scientific understandings of sex differences, sexual behavior, and reproduction; attitudes toward homosexuality, transgenderism, and other nonconforming sexualities; the ways that sexual conduct was governed, and that government was sexualized; the sometimes complementary, sometimes antagonistic relationship between sexuality and religion; the sexual dimensions of colonialism and imperial expansion; and the ever-shifting purposes, markets, and definitions of obscene media. Each chapter also explores ways in which the sexual ambivalences of the Enlightenment persist to the present day.