In the late 1960s and early 1970s, a conceptual distinction between “sex” and “gender” arose in the clinical literature on human psychosexual development. Sex came to signify the biological or bodily component of difference, that is, male and female. Gender, on the other hand, came to signify the social or cultural component of difference, that is, masculine and feminine. This sex/gender distinction, as it is often called, was heartily embraced by many feminists of the day who sought to account for differences between the sexes as well as explain and remediate women’s second-class status in society. The establishment of gender as a distinctly “social” concept appealed to feminists because it opened up an intellectual and political space—a space beyond biological determinism—for inquiry into the causes of “male domination” and “female subordination” that were not essential, universal, or fixed. In this space, social change was possible; gender relations could be reconfigured. To that end, the sex/gender distinction became, by and large, paradigmatic in feminist thought and social science, and from it grew a burgeoning body of gender theory loosely characterized as the social construction of gender. Intersectional, post-structural, postmodern, and queer schools of thought produced new insights and advanced theory in ways that posed challenges to the viability and utility of gender as a concept as well as to the sex/gender paradigm. The ensuing debates were highly productive, ushering in a new era of social theory on the body that centered corporeality and embodiment and that sought to deconstruct binary thinking. As thinking on sex/gender evolved, the conceptual split was no longer understood as a simple separation between the biological and the social. Feminist and queer scholars problematized the distinction, reformulating it as an interlocking set of relationships: the sex/gender/sexuality system. Interdisciplinary gender scholars, including prominent feminist scientists, began theorizing the complex interrelationship between sex and gender with greater sophistication in an attempt to more firmly discredit biological determinist approaches to the study of difference based on sex, gender, or sexuality. Advancing theory, research, and praxis has not only deepened understanding about a wider variety of identities, experiences, and practices around sex, gender, and sexuality but has also won greater recognition in the early 21st century for them. This multiplicity of sexes, genders, and sexualities has brought with it unique methodological concerns in the social sciences, which represent a new frontier of research and activism in gender and sexuality studies.