In this paper we report on the development of an inventory that measures global citizenship among adolescents. The methodology used consists of cognitive interviews for questionnaire design and explorative and confirmatory factor analyses among several datasets. The resulting Global Citizenship Inventory (GCI) includes a global citizenship behaviour index and three scales measuring principles underlying global citizenship: human equality, mutual dependency in the world, and the shared responsibility for solving global issues. The behaviour index measures behaviour connected to efficient use of energy and water, mobility, recycling and handling waste, search for information and expressing an opinion on global issues, donating to charity, and volunteering. The results provide support for our theoretical constructs regarding global citizenship.