Material Girls presents a timely and opinionated critique of the culturally influential theory that we each have an inner feeling about our sex called a gender identity, and that this feeling is more socially significant than our actual biological sex. It makes a clear and humane feminist case for retaining the ability to discuss material reality about biological sex in a range of important contexts, including female-only spaces and resources, healthcare, epidemiology, political organization and data collection. It investigates the intellectual history of gender identity, showing how the concept is linked to a misguided philosophical picture which broadly rejects science and conflates facts about intersex people with facts about trans people. Material Girls concludes with a positive vision for the future, of collaboration between feminists and trans activists, detailing how they could work together to achieve some of their political aims.