What role does data play in addressing gender-based violence? This report summarizes the results of five years of collective work as part of the Data Against Feminicide project. Data Against Feminicide is a South-North participatory action research and design project that supports the work of activists and civil society organizations that produce data on gender-based violence and its most lethal expression: feminicide. The report brings together questions and concepts that underpin the project, shares what we have learned about data activism against feminicide in various contexts – particularly in Latin America and North America – and presents technological tools that we have developed in collaboration with several activists to support the production of data on feminicide.