The term "femme" originates from 1940s Western working-class lesbian bar culture, wherein femme referred to a feminine lesbian who was typically in a relationship with a butch lesbian. Expanding from this original meaning, femme has since emerged as a form of femininity reclaimed by queer and culturally marginalized folks. Importantly, femme has also evolved into a theoretical framework. Femme theory argues that "femme" constitutes a missing piece in queer and feminist discourses of femininity. Attending to this gap, femme theory centres queer femininities as a means of pushing against the deeply embedded masculinist orientation of queer and gender theory. Thus, femme theory offers tools to shift the way researchers and readers understand femininity as well as systems of gender and power more broadly. This book is an introduction to femme theory, showcasing how femme can be used as a theoretical framework across a variety of contexts and disciplines, such as Film & Media Studies, Psychology, Sociology, or Critical Disability Studies; from countries, including Canada, China, Guyana and the USA. Femme theory asks readers to reconsider how femininity is conceptualized, revealing some of the many taken for granted assumptions that are embedded within cultural discourses of gender, sexuality, and power. The chapters in this book were originally published as a special issue of the Journal of Lesbian Studies.