Global citizenship education is transformative in the sense that it facilitates learners in developing a sense of their societal roles and identities at the local, national, and global levels as well as in participating at these interconnected levels using their cognitive, socio-affective, and behavioral skills. Agency, the socioculturally mediated capacity to act (Ahearn 2001), is at the center of these skills and participatory levels. It involves initiative and self-regulation by the learner, his or her awareness and responsibility for one’s own actions vis-à-vis the sociocultural environment (van Lier 2009), and frequently is a property of social groups, arising from mutual engagement (Wertsch et al. 1993). Yet, agency is a fuzzy concept that defies direct quantification for both empirical and pedagogic purposes (Mercer 2012). Current EFL research, therefore, is in need of a model of learner agency that can inform EFL theory, educational planning and implementation, and diagnostic and evaluative procedures. We investigate the development of learner agency in the context of “Going Green – Education for Sustainability”, a German-American blended learning project for the EFL, social studies, and bilingual STEM classrooms that asks students to challenge commonly held stereotypes about how both cultures approach the question of sustainable development. Over 2,500 secondary school students have successfully participated since 2014. Every year, students collaboratively complete a blended-learning, task-based curriculum, develop green action plans for their schools and communities, publish them online, and present them at a national student conference. We discuss a research agenda and exemplary results of the project Fremdsprache – Fachsprache – Agency, a two-tier, mixed-methods study on the development of STEM-based discourse competence and sociopolitical participation in the aforementioned project context. We argue that learner agency draws on a set of skill areas, including foreign language discourse competence in English as a global lingua franca, digital literacies, (subject-specific) science literacy, and critical literacy. References Ahearn, L. M. (2001). Language and Agency. Annual Review of Anthropology, 109-137. Mercer, S. (2012). The Complexity of Learner Agency. Apples - Journal of Applied Language Studies, 6, 41–59. https://doi.org/10.1002/9781118170229.ch1 van Lier, L. (2008). Agency in the classroom. In J. P. Lantolf & M. E. Poehner (Eds.), Studies in applied linguistics. Sociocultural Theory and the Teaching of Second Languages (pp. 163–186). London: Equinox. Wertsch, J. V., Tulviste, P., & Hagstrom, F. (1993). Sociocultural Approach to Agency. In E. A. Forman, N. Minick, & C. A. Stone (Eds.), Contexts for Learning: Sociocultural Dynamics in Children's Development (pp. 336–356). Oxford: Oxford Univ. Press.