This Series paper describes programming to prevent violence against women and girls, and emphasises the importance of systematic, sustained programming across the social ecology (ie, the delicate equilibrium of interacting social, institutional, cultural, and political contexts of people's lives) to transform gender-power inequalities. Effective prevention policy and programming is founded on five core principles: first, analysis and actions to prevent violence across the social ecology (individual, interpersonal, community, and societal); second, intervention designs based on an intersectional gender-power analysis; third, theory-informed models developed on the basis of evidence; fourth, sustained investment in multisector interventions; and finally, aspirational programming that promotes personal and collective thought, and enables activism on women's and girls' rights to violence-free lives. Prevention programming of the future will depend on all of us having a vision of, and a commitment to, gender equality to make violence-free lives for women and girls a reality.