The world faces an unprecedented environmental crisis, and human activity is its root cause. This poses a call to action for psychology and the social sciences more broadly. GREENTEENS proposes that our adolescents—a segment of the population whose collective behavior will shape the future of the planet—can play a vital role in creating a more sustainable world. Adolescents, however, are “closet idealists”: As a group, they care about the environment but often fail to act on their concerns. The aim of the proposed research is to develop a new approach to understanding and promoting adolescents’ eco-friendly behavior. It will generate new understanding of what keeps adolescents from engaging in eco-friendly behavior, and devise methods to help youth contribute to a sustainable future for themselves and generations to come. I have developed a new hypothesis for this project: the “Motive-Match Hypothesis”. It casts adolescents’ eco-friendly behavior as driven by their personal motives. Based on the hypothesis, I will design methods to transform the way adolescents construe eco-friendly behavior, from a low-priority chore to an activity that embodies what they deeply care about—developing autonomy and gaining peer status. Building on my international network, I will pursue the research aim using a cross-national investigation involving adolescents (age 12-17) from The Netherlands, Colombia, and China. The project will integrate longitudinal research to understand how adolescents’ eco-friendly behavior develops over time, with experiments to understand how adolescents’ core motives can be harnessed as powerful motivating force for eco-friendly behavior. GREENTEENS will advance the science of adolescent behavior change beyond the state of the art. The payoff of the research promises to be high: It will yield fundamental understanding of what drives adolescents’ eco-friendly behavior and help improve pro-environmental policies targeting millions of youth worldwide.