Sustainable development is a prominent issue on the agendas of citizens, policymakers, practitioners, and scholars alike. Conceptual frameworks have been suggested to guide teaching-learning processes in order to prepare future generations for sustainability challenges. One such framework is action competence in sustainable development (ACiSD) as a learning outcome of action-oriented education for sustainable development (ESD). The current chapter introduces the main components of this ACiSD-ESD conceptual framework for its operationalization using data from international large-scale assessments. ACiSD is defined by the acquisition of relevant knowledge and skills, a strong motivation from within, and confidence both in one’s own capacities for inducing change and in a positive outcome of sustainability actions. ESD is characterized by holism, pluralism and participation, and an orientation toward action. Currently, international large-scale assessments such as the Trends in International Mathematics and Science Study (TIMSS) and International Civic and Citizenship Education Study (ICCS) are evolving towards providing opportunities to operationalize the ACiSD-ESD conceptual framework. This may enable monitoring ESD learning outcomes and sharing good practices on an international basis.