The history of women's exclusion and invisibility in cities is well charted, yet young women's experience of sexual harassment and assault has been difficult to quantify. This article discusses the Free to Be project initiated by Plan International in 2018. In partnership with Monash University's XYX Lab and CrowdSpot, the crowdmapping web app enabled young women in Delhi, Kampala, Lima, Madrid, and Sydney to identify and share their experiences of public spaces. It is believed to be the most ambitious global crowdsourced data collection project on street harassment ever undertaken. By using coding and visual data techniques, the data demonstrated the scale of the problem and the urgent need for city-level decision makers, as well as other members of society, to take action. This article outlines the findings from Free to Be, and explores the ways data and activism led by girls and young women are a powerful method for creating change.