The United Nations' Goals for Sustainable Development highlight gender inequality as a pervasive problem around the world. Developmental psychologists can help us understand the development and consequences of sexism in people's lives. I highlight ambivalent sexism theory as a promising framework for this work; and I offer recommendations for expanding the theory. Ambivalent sexism theory distinguishes between hostile sexism and benevolent sexism as complementary processes perpetuating and maintaining men's dominance and heteronormativity in society. I summarize how these two forms of sexism emerge during childhood and adolescence; and I review the negative manifestations of hostile and benevolent sexism during adolescence and adulthood. Next, I chart several directions for expanding the ambivalent sexism model. These include addressing sexism directed towards gender-nonconforming, sexual-minoritized, and gender-minoritized youth (in addition to sexism towards girls and women); taking into account the gender and sexual identities of both perpetrators and targets of sexism; considering a broader array of hostile and benevolent sexist practices than captured in existing measures; taking into account cultural variations and intersectionality in how ambivalent sexism is enacted; conducting more research on ambivalent sexism in childhood and adolescence and designing effective programs to reduce and to prevent ambivalent sexism beginning in childhood.