This article encourages sociologists to study gender as a social institution. Noting that scholars apply the institution concept to highly disparate phenomena, it reviews the history of the concept in twentieth-century sociology. The defining characteristic most commonly attributed to social institution is endurance (or persistence over time) while contemporary uses highlight practices, conflict, identity, power, and change. I identify twelve criteria for deciding whether any phenomenon is a social institution. I conclude that treating gender as an institution will improve gender scholarship and social theory generally, increase awareness of gender's profound sociality, offer a means of linking diverse theoretical and empirical work, and make gender's invisible dynamics and complex intersections with other institutions more apparent and subject to critical analysis and change.