The environmental Kuznets hypothesis predicts that pollution will increase at early stages of development but then decline once a country surpasses a certain income level. This column examines how banks and stock markets affect the mechanisms behind this hypothesis. Industries which pollute relatively more for technological reasons generate relatively more carbon dioxide in countries with expanding credit markets, whereas stock markets have the exact opposite effect. For middle-income countries in particular, where carbon dioxide emissions may have increased linearly during the development process, stock markets could play an important role in making future growth greener.